Velocity in Bolt action combarred to semi-auto

[rwt101]

Just wondering if a bolt action rifle has more velocity than a semi-auto? That is using the same ammo and same length barrel. I thought that the bolt action would be higher because in the semi auto it has to cycle the action. Any thoughts on it?
Bob T

[Rocky Raab]

Nothing in a gas auto moves until well after the bullet has left the barrel - by design. The very tiny volume of gas that is bled off into the piston or other system is so small that it accounts for less than the variation in pressure between any two shots.

So any difference in velocity between the two gun types is due to some other factor, of which there are many.

[Bucks Owin]

I expect the difference in action types wouldn't even affect velocity as much as "Standard deviation/Extreme spread" with equal length barrels.....JMO, Dennis :coffeecom (Variations in chamber/throat dimensions would have a far greater impact than the action itself IMO))

[44man]

True, there is no difference at all.

[StarMetal]

Nothing in a gas auto moves until well after the bullet has left the barrel - by design. The very tiny volume of gas that is bled off into the piston or other system is so small that it accounts for less than the variation in pressure between any two shots.

So any difference in velocity between the two gun types is due to some other factor, of which there are many.

I don't know about that Rocky. Ever heard it said "the action started to open before all the pressure was gone"??

At any rate I can find out by shooting some 223 over my chrono with AR15 with the gas system on and then off.

Joe

[theperfessor]

Starmetal-
Good idea. Nothing beats a real world test. One fact is worth a thousand opinions. Let us know results.

[Rocky Raab]

Ahh, but there is significant pressure left for a good amount of time after the bullet has left the barrel. Look at any pressure trace, and see that pressure does not drop to zero instantly - it tapers off rapidly, but not instantly. (Time here is relative, of course. The whole sequence takes only a couple or few thousandths of a second from primer hit to bullet exit.)

Residual pressure is what makes blowback actions work, what makes the 1911 work, and what makes gas autos work. And it is all based on pressure remaining after the bullet is gone.

[pdawg_shooter]

Ahh, but there is significant pressure left for a good amount of time after the bullet has left the barrel. Look at any pressure trace, and see that pressure does not drop to zero instantly - it tapers off rapidly, but not instantly. (Time here is relative, of course. The whole sequence takes only a couple or few thousandths of a second from primer hit to bullet exit.)

Residual pressure is what makes blowback actions work, what makes the 1911 work, and what makes gas autos work. And it is all based on pressure remaining after the bullet is gone.

I do believe recoil makes the 1911 cycle, not pressure!

[mike in co]

I don't know about that Rocky. Ever heard it said "the action started to open before all the pressure was gone"??

At any rate I can find out by shooting some 223 over my chrono with AR15 with the gas system on and then off.

Joe
true opening before the pressure is gone, but not before the bullet is gone.....
ohhh(rocky beat me to it)

mike in co

[mike in co]

I don't know about that Rocky. Ever heard it said "the action started to open before all the pressure was gone"??

At any rate I can find out by shooting some 223 over my chrono with AR15 with the gas system on and then off.

Joe

joe...shoot a string, then remove the gas block with the tube attached. install a new gas block that is blocked or turned 90 degrees.

takes some kroil with you ...carbon build up make it all a tight fit.

mike in co

[KYCaster]

I do believe recoil makes the 1911 cycle, not pressure!


I think you're right.

Jerry

[felix]

No pressure, no recoil. Therefore, you can readily say some pressure is bled from the boolit to make the gun eject. Heavier the recoil springs, the more pressure is allocated to the boolit. ... felix

[leftiye]

Yep, pressure causes the boolit to leave, and causes the "equal and opposite" recoil while doing so. It may (high probability Remo) still be that the boolit is long gone before the slide moves, and/or the breech opens.

[StarMetal]

joe...shoot a string, then remove the gas block with the tube attached. install a new gas block that is blocked or turned 90 degrees.

takes some kroil with you ...carbon build up make it all a tight fit.

mike in co

Mike I don't have to do that, I can just turn the gas valve off which directly behind the block.

Joe

[StarMetal]

Rocky,

I can guarantee you there is gas in the carrier/bolt cylinder/piston before that bullet is gone.

Joe

[StarMetal]

I think you're right.

Jerry

All semi auto pistols that do not have a gas system are blow back operated. Those with a lockup such as the browning system are a delayed blow back.

Joe

[yondering]

No pressure, no recoil. Therefore, you can readily say some pressure is bled from the boolit to make the gun eject. Heavier the recoil springs, the more pressure is allocated to the boolit. ... felix

Felix, your post seems to imply that recoil is created by pressure, or related in some way. This is not true. Recoil is the reaction to the projectile's inertia (and to some small degree, the jet effect of the escaping gasses). Higher pressure does not equate to higher recoil. Some projectile launching systems, like a slingshot or a bow, experience no chamber pressure of course, yet they still have recoil.

Recoil operates the slide on a 1911, NOT pressure. Most .22 semi-auto's are operated by pressure (blowback action), a 1911 or most other locked breech semi-auto pistols are completely different.

[StarMetal]

Felix, your post seems to imply that recoil is created by pressure, or related in some way. This is not true. Recoil is the reaction to the projectile's inertia (and to some small degree, the jet effect of the escaping gasses). Higher pressure does not equate to higher recoil. Some projectile launching systems, like a slingshot or a bow, experience no chamber pressure of course, yet they still have recoil.

Recoil operates the slide on a 1911, NOT pressure. Most .22 semi-auto's are operated by pressure (blowback action), a 1911 or most other locked breech semi-auto pistols are completely different.

Felix has it right. I think you, like many, get a locked system like the 1911 confused as being different then a blow back one. It's a blow back system, only delayed. When the cartridge is fired combust creates the gases and pressure. The pressure is trying to do two things. One is push the bullet down the bore and the other is to try push the case back. Being the case obturates, in a pure blow back, it pushes the slide/barrel assembly back. In the 1911 it does this too for about 1/8 inch and the toggle link cams the barrel lugs out of the locked position. It's the back thrust, created from the gas pressure, that initially started the whole assembly back and it's the inertia and the little bit of remaining gas that continue it's push back.

Recoil is the results of the combined reaction to pushing out the projectile, the powder, and some from the gases as you mentioned.

Joe

[felix]

For the bow operation, the pressure comes from your arm pulling back the string. In a firearm, the pressure comes from expanding gas. The question arises to how much pressure is bled from the projectile in either case during firing, either gun or bow. ... felix

[mike in co]

Rocky,

I can guarantee you there is gas in the carrier/bolt cylinder/piston before that bullet is gone.

Joe
joe...how you gonna do that ?
there is air/gas from the last shot...no significant pressure.....
so how much are you talking about ???

20 bbl 55 fmj 3100 fps......
bullet exits in 0.95ms
pressure at the port starts at .74ms
that means pressure has .21ms at a starting pressure of 19500 to make it back to the carrier.

some pressure...maybe...enought to move anything...not likely..but i'll listen

mike in co

[AZ-Stew]

Well, let's see...

The US Military called the Garand, M-1 Carbine, M-14, M-16 (and variants) "Gas operated".

The 1911 .45 ACP and the M-2 Browning (as well as several other machine guns) "Recoil operated".

The M-3 Grease Gun and, I believe, the Thompson SMG were classified as "Blow-back". The same action type is used in most .22 semi-autos.

In Gunner's Mate school we were taught that the difference between the recoil operated and blow-back actions was the locked breech. Blow-back actions rely upon the mass of the breech block and the return spring force to keep the breech closed until the bullet leaves the barrel and gas pressure in the barrel was low enough to allow safe opening of the action. With the recoil operated actions, the breech is mechanically locked until the bullet leaves and pressure drops. In both cases, the action parts have started to move before the bullet leaves the barrel, but the case is not fully extracted until after it has. If you've ever seen the parts of the grease gun, you'll know that the breech block has FAR more mass than the slide of the 1911. If the pistol didn't have the locked breech it would throw the slide through the shooter's face on the first round. So the 1911 is not a blow-back. Call it a "delayed" blow-back if you want, but the military calls it a recoil operated action. They're different.

My $0.02, but since I don't have access to any pressure measurement tools or the ability to detect when in the pressure curve the bullet actually exits the muzzle, I'll leave the remainder of the discussion to those who do. I just wanted to clear up the differences in the actions. There seemed to be some misconceptions.

Regards,

Stew

[StarMetal]

Well, let's see...

The US Military called the Garand, M-1 Carbine, M-14, M-16 (and variants) "Gas operated".

The 1911 .45 ACP and the M-2 Browning (as well as several other machine guns) "Recoil operated".

The M-3 Grease Gun and, I believe, the Thompson SMG were classified as "Blow-back". The same action type is used in most .22 semi-autos.

In Gunner's Mate school we were taught that the difference between the recoil operated and blow-back actions was the locked breech. Blow-back actions rely upon the mass of the breech block and the return spring force to keep the breech closed until the bullet leaves the barrel and gas pressure in the barrel was low enough to allow safe opening of the action. With the recoil operated actions, the breech is mechanically locked until the bullet leaves and pressure drops. In both cases, the action parts have started to move before the bullet leaves the barrel, but the case is not fully extracted until after it has. If you've ever seen the parts of the grease gun, you'll know that the breech block has FAR more mass than the slide of the 1911. If the pistol didn't have the locked breech it would throw the slide through the shooter's face on the first round. So the 1911 is not a blow-back. Call it a "delayed" blow-back if you want, but the military calls it a recoil operated action. They're different.

My $0.02, but since I don't have access to any pressure measurement tools or the ability to detect when in the pressure curve the bullet actually exits the muzzle, I'll leave the remainder of the discussion to those who do. I just wanted to clear up the differences in the actions. There seemed to be some misconceptions.

Regards,

Stew

The 1911 is a blowback action:

http://www.guncollectorsclub.com/blowback.htm

Once more powerful ammunition is used, some other mechanism has to be
employed to ensure that the pressures produced fall to a safe level before the
fi red cartridge case exits from the barrel. This is accomplished via a locked
breech or delayed blowback mechanism in which the barrel is locked to the
breech block by some mechanical means during the instant of fi ring.
With this type of action, the rearward thrust of the cartridge case against the
breech block causes the barrel and attached breech block to move backwards
together. At some point on its rearward travel, designed such that the bullet has
exited the barrel and the barrel pressures have fallen to acceptable levels, the
barrel is stopped and unlocked from the breech block. The breech block and slide
can then continue to the rear and in so doing eject the empty cartridge case.

There are all kinds of delayed blowback actions. The blish-block in the early Thompson are considered a retarded blowback action.

Joe

[Uncle R.]

The 1911 is a short recoil action because the barrel actually moves rearward for less than the length of the cartridge. If the barrel moved rearward for more than the length of a cartridge like an Auto 5 shotgun it would be a long recoil action. If the barrel were fixed and the bolt was moved rearward by the cartridge case alone it would be a blowback. One way to test is to remove the extractor and check function. Blowback actions function perfectly without their extractors - the only purpose of the extractor is to unload a live cartridge. Recoil actions function poorly or nor at all without their extractors. Yes - I've shot 1911s with broken extractors (or tried to) and they didn't work well.
All this based on what I've read over the years. You guys can argue all you want - enjoy yourselves!
<GRIN>
Uncle R.

[KYCaster]

.....while we hijack your thread.

I suppose you could argue that the Mauser M98 and the Colt's Model P are both "gas opperated" since it is gas pressure that pushes the bullet down the barrel, but I don't think that dog will hunt.

Correct me if I'm wrong but doesn't a "gas opperated" gun have to bleed gas out of the barrel into a seperate tube or chamber and use that gas to power the mechanism that completes the firing cycle?

I've owned a few 1911's and none of them had a "GAS TUBE" or "OP ROD" but all of them had a "RECOIL SPRING". And none of them had any ports in the barrel to bleed off gas to opperate the system.

When the bullet leaves the case there is that "equal and opposite reaction" thing (RECOIL) that tries to push the case the other way....but there's a breech face in the way that's part of the slide that's keyed to the barrel so it has to move the whole sheeebang against the RECOIL SPRING to complete the firing cycle.

And "blow back" or "delayed blow back" are just sub-catagories of recoil opperated designs. Neither of them uses the combustion gas in any way to complete the firing cycle...they're opperated purely by recoil.

Jerry

[Rocky Raab]

A true blowback action is not locked at all. It is held and prevented from moving by the force of the brass case being held tightly to the chamber by propellant gas. After the bullet leaves the muzzle, the gas pressure begins to drop. At a certain point, the dropping pressure allows the brass case to release its grip, but there is still quite a lot of pressure, and that is what causes the case to start backward, pushing the breechblock/slide and recoil springs as it does so. Examples are .22 rimfire semi-autos, most .380 pistols, Makarovs, and the M-3 "grease gun."

Locked-barrel actions work in much the same way, except that the barrel is not stationary and is locked to the slide (not depending on case adhesion alone). As above, as the pressure drops following bullet exit, the case releases its grip. The slide and barrel move rearward until the barrel disengages its locking lugs, and thereafter the slide alone continues rearward. Examples are the 1911 pistol, the Glock and many other handguns, the Browning Auto-5 shotgun and others.

Gas-operated actions use gas that is tapped off from the bore, at some distance from the breech (and often near the muzzle). The action is locked closed until that gas pushes on a piston or directly on the breechblock to begin unlocking the action. But once again, NO motion begins until after the bullet is long gone.

The delay in all systems described is as long as five thousandths of a second AFTER the bullet leaves the muzzle.

[StarMetal]

The 1911 is a short recoil action because the barrel actually moves rearward for less than the length of the cartridge. If the barrel moved rearward for more than the length of a cartridge like an Auto 5 shotgun it would be a long recoil action. If the barrel were fixed and the bolt was moved rearward by the cartridge case alone it would be a blowback. One way to test is to remove the extractor and check function. Blowback actions function perfectly without their extractors - the only purpose of the extractor is to unload a live cartridge. Recoil actions function poorly or nor at all without their extractors. Yes - I've shot 1911s with broken extractors (or tried to) and they didn't work well.
All this based on what I've read over the years. You guys can argue all you want - enjoy yourselves!
<GRIN>
Uncle R.

Uncle R.

In a 1911 the barrel never moves but two ways and that is up and down. The barrel is carried back and forth by the slide.

Joe

[StarMetal]

A true blowback action is not locked at all. It is held and prevented from moving by the force of the brass case being held tightly to the chamber by propellant gas. After the bullet leaves the muzzle, the gas pressure begins to drop. At a certain point, the dropping pressure allows the brass case to release its grip, but there is still quite a lot of pressure, and that is what causes the case to start backward, pushing the breechblock/slide and recoil springs as it does so. Examples are .22 rimfire semi-autos, most .380 pistols, Makarovs, and the M-3 "grease gun."

Locked-barrel actions work in much the same way, except that the barrel is not stationary and is locked to the slide (not depending on case adhesion alone). As above, as the pressure drops following bullet exit, the case releases its grip. The slide and barrel move rearward until the barrel disengages its locking lugs, and thereafter the slide alone continues rearward. Examples are the 1911 pistol, the Glock and many other handguns, the Browning Auto-5 shotgun and others.

Gas-operated actions use gas that is tapped off from the bore, at some distance from the breech (and often near the muzzle). The action is locked closed until that gas pushes on a piston or directly on the breechblock to begin unlocking the action. But once again, NO motion begins until after the bullet is long gone.

The delay in all systems described is as long as five thousandths of a second AFTER the bullet leaves the muzzle.

Rocky,

On the 1911 the barrel and slide start rearward at the moment of firing. They are traveling about 1/8 of a inch at which time the bullet is long gone and the case releases it's grip...that's when the barrel can unlock and the slide travel on it's way.

Joe

[Multigunner]

Using a powder that burns too slowly can result in a higher gas port pressure, the pressure not having dropped enough at the point when the bullet passes the gas port.

I've seen FN FAL rifles nearly rip the rims off US Matchgrade 7.62 ammo which used either Winchester or a similar double based powder anf a heavy bullet. This can happen with the regulator turned down as far as it can go and still cycle.

The problem comes from the oprod and piston moving back faster than normal and trying to open the action while the case is still fully expanded, when it does kick open the grip on the chamber walls is still great in comparasion to the rapidly dropping pressure in the bore.
A similar problen occurred with the M16 when Ball powder was substituted for IMR powder for the 5.56. The powder used gave too high a gas port pressure increasing rate of fire and sometimes causing case separations should there be enough fouling in the chamber to increase the grip of the case on the chamber walls.

Pistol powders used for the .45 ACP burn too fast and the barrel is much too short for there to be much if any residual pressure in the case when the barrel unlocks. If there were the chamber would have had to be fluted (like the CETME or G3) or fitted with a plenium chamber (as are some .22 Magnum blowback or delayed blowback actions) to float the case on gas, or the cases waxed, like the cases of most delayed blowback weapons are.

When slow powders and heavy bullets are used with the Garand the op rod can sometimes bend or break because its trying to open the action while chamber pressure is still too high.

[S.R.Custom]

Just wondering if a bolt action rifle has more velocity than a semi-auto?

To answer your question, yes. Physics tells us that all else being equal, a bolt gun will always deliver more velocity than a semi-auto. In some instances the efficiency of the mechanism may be such that the difference in velocity is so small as to be able to dance on the head of a pin, but it is there nonetheless.

And that's what time it is. I now return you to the discussion on how to build a clock....

[mike in co]

To answer your question, yes. Physics tells us that all else being equal, a bolt gun will always deliver more velocity than a semi-auto. In some instances the efficiency of the mechanism may be such that the difference in velocity is so small as to be able to dance on the head of a pin, but it is there nonetheless.

And that's what time it is. I now return you to the discussion on how to build a clock....

go read the math of the speed of events in an ar...and rethink your opinion


mike in co

[felix]

SuperMag is correct, Mike. But, in the real world of gun tolerances, your gun might be the fastest in town. ... felix

[StarMetal]

Using a powder that burns too slowly can result in a higher gas port pressure, the pressure not having dropped enough at the point when the bullet passes the gas port.

I've seen FN FAL rifles nearly rip the rims off US Matchgrade 7.62 ammo which used either Winchester or a similar double based powder anf a heavy bullet. This can happen with the regulator turned down as far as it can go and still cycle.

The problem comes from the oprod and piston moving back faster than normal and trying to open the action while the case is still fully expanded, when it does kick open the grip on the chamber walls is still great in comparasion to the rapidly dropping pressure in the bore.
A similar problen occurred with the M16 when Ball powder was substituted for IMR powder for the 5.56. The powder used gave too high a gas port pressure increasing rate of fire and sometimes causing case separations should there be enough fouling in the chamber to increase the grip of the case on the chamber walls.

Pistol powders used for the .45 ACP burn too fast and the barrel is much too short for there to be much if any residual pressure in the case when the barrel unlocks. If there were the chamber would have had to be fluted (like the CETME or G3) or fitted with a plenium chamber (as are some .22 Magnum blowback or delayed blowback actions) to float the case on gas, or the cases waxed, like the cases of most delayed blowback weapons are.

When slow powders and heavy bullets are used with the Garand the op rod can sometimes bend or break because its trying to open the action while chamber pressure is still too high.

I believe the G3, for example, had a fluted chamber to assist extraction. HK claimed it would throw the empty case out without an extractor.

Joe

[mike in co]

SuperMag is correct, Mike. But, in the real world of gun tolerances, your gun might be the fastest in town. ... felix
me thinks not....go do your test....

the bullet is passed being influnced by the pressure drop...my 2 cents worth.



mike in co

[drinks]

I tried this at least 50 years ago, in a .22 rf , self loaders will not achieve the same depth of penetration in wood as a single shot or bolt action, the self loaders were a Rem. 550A and a Win. 63, this was done at the same time and with the same ammunition out of the same box.
I have no information on gas operated self loaders.

[StarMetal]

I tried this at least 50 years ago, in a .22 rf , self loaders will not achieve the same depth of penetration in wood as a single shot or bolt action, the self loaders were a Rem. 550A and a Win. 63, this was done at the same time and with the same ammunition out of the same box.
I have no information on gas operated self loaders.

Were the barrel lengths all the same? That makes a big difference in 22's. Also just different brand rifles or ever different rifle have a wide variance.

I think the better test would be a centerfire semi auto rifle with a gas valve.

Joe

[Ed Barrett]

Next time I'm at the range I'll try my Yugo SKS with the gas valve turned on and off and run it over the chrono. Quick test. Maybe if anyone has an M-1 with a gernade firing gas plug that would be another way, or just remove the plug this would let the gas lost through the pressure bleed off hole go into the air with nothing to slow it.

[Bass Ackward]

Back in 1978 a buddy sort of .... appropriated some extra military ball to shoot in his 223. The ammo chambered beautifully but the first shot ruptured the case, ruined the stock, and stuck the bolt badly.

So some pressure has to be going somewhere in a "gas operated" semi before the bullet exits that makes the loads safe.

[StarMetal]

Back in 1978 a buddy sort of .... appropriated some extra military ball to shoot in his 223. The ammo chambered beautifully but the first shot ruptured the case, ruined the stock, and stuck the bolt badly.

So some pressure has to be going somewhere in a "gas operated" semi before the bullet exits that makes the loads safe.

Bass,

He done exactly what we're told not to do, not to interchange NATO 5.56 and .223 ammo. Now I may be wrong about your story as you didn't elaborate on what type and brand rifle he fired it in.

Joe

[pmeisel]

Those of you doing home tests -- do at least 30 shots of each under otherwise identical conditions... I'll dig out the formulas to test for statistical significance.

I would bet against statistical significance but if we all had enough interesting stuff to do we wouldn't be reading posts on the internet, would we?

[mike in co]

I tried this at least 50 years ago, in a .22 rf , self loaders will not achieve the same depth of penetration in wood as a single shot or bolt action, the self loaders were a Rem. 550A and a Win. 63, this was done at the same time and with the same ammunition out of the same box.
I have no information on gas operated self loaders.

not the same test at all, as 22 are typically blow back low velocity guns.....not like a centerfire rifle at all.


mike in co

[mike in co]

Back in 1978 a buddy sort of .... appropriated some extra military ball to shoot in his 223. The ammo chambered beautifully but the first shot ruptured the case, ruined the stock, and stuck the bolt badly.

So some pressure has to be going somewhere in a "gas operated" semi before the bullet exits that makes the loads safe.



wrong....but there are two different chambers .......the 5.56 being more forgiving and the 223 has the potenial for failure with 5.56 ammo...as you buddy found out.

(which is why most ar's are built with a 5.56 chambers/or target version of it)
mike in co

[felix]

Mike, I don't test anything unless it is for MY application, and even then I don't call it testing, but honing in instead. In other words, I don't engage in "what if" scenarios any more. Sorry. ... felix

[Bass Ackward]

wrong....but there are two different chambers .......the 5.56 being more forgiving and the 223 has the potenial for failure with 5.56 ammo...as you buddy found out.

(which is why most ar's are built with a 5.56 chambers/or target version of it)
mike in co


Wrong? Different chambers? Mike, come on. Your pullin my leg here right? :grin:

Back in olden times, the 55 grain bullet of the ball ammunition was loaded to 60,000 CUP while the 223 was loaded to 50,000 CUP.

Both cartridges produce 3200 fps with that weight ball and length barrel.

And the actions of the M16 carboned up for a reason.

[StarMetal]

Read this and see what I mean there may be gas at the carrier/bolt before the bullet leaves the barrel:

Ok, starting with a cartridge in the chamber, hammer back.
Trigger lets the hammer fall.
Hammer hits the firing pin, driving it forward.
Firing pin drives the primer (and attached cartridge case) forwards in the chamber until the shoulder in the chamber stops the shoulder on the cartridge case.
The case will already be seated against the shoulder due to ejector tension, but the primer can sometimes move before the anvil legs on the primer stop against the primer pocket.


Headspace gauges account for the length of the cartridge AND for the recommended amount of headspace, but what really matters is the amount of space, or lack there-of, of space between the case head and bolt face.
You need some headspace or there will not be enough room for the bolt lugs to lock into engagement if you get any dirt or drit on your ammo. You don't want too much space though or you run into problems with brass stretching or ignition problems and case failures in extreme cases.
The firing pin continues forward to ignite the primer.
Primer flash ignites powder charge, instantly creating great pressure within the cartridge case.
Cartridge case expands first outward towards chamber walls (path of least resistance) where pressure holds the case in place and then the case stretches backward until the case head is stopped against the bolt face.
That is WHY long headspace makes cases fail!
It's normal for the brass to stretch rearward a couple thousandths and the brass will spring back when chamber pressure subsides. You don't want to have enough headspace for the brass to yield (permanently stretch) as the case gets thin just above the extraction groove where the walls thin out. Excessive headspace shows up on the cases as a shiney ring about 1/8" forward of the extraction groove.
Bullet begins movement down the barrel, first encountering the throat.
Here is why you want a throat DIAMETER closely matching the bullet.
Loose throats do not control the bullet and keep it as straight while engraving into the rifling.
More about throat dimensions can be found here. (http://www.ar15barrels.com/data/223-556.pdf)
As the bullet starts down the barrel, it jams into the rifling where pressure causes the rifling lands to "engrave" into the bullet. Dependng on the aspect ratio of the lands to grooves, the bullet will sometimes grow in length. This radical change in bullet shape can often be detrimental to accuracy.
Ideally, you would want a barrel with an odd number of lands and grooves.
This allows the bullet to obturate more gently.
Now the bullet has obturated and engraved into the rifling and it's accelerating rapidly down the bore.
As it passes the gas port, gas begins to flow into the gas block where it turns and heads towards the bolt carrier via the gas tube.
The pressure is still high in the barrel, usually 15,00PSI+ until the bullet leaves the muzzle.
Just as the bullet leaves the muzzle, gas escapes around the base of the bullet.
Here is why a proper crown is important.
Gas is traveling about 5x faster than the bullet when it leaves the muzzle.
An even crown releases gas all the way around the bullet at one time.
An un-even crown lets gas go on one side first.
This can tip the bullet just slightly sideways at the moment the bullet is released into the air.
This is a very important time in the bullet's flight.
Now, remember, high pressure gas always follows the path of least resistance, which is now out the front of the barrel instead of into the gas system.
Barrel pressure drops immediatly.
During the bullet's travel down the bore between the gas port and the muzzle, we had a metered amount of gas fed to the action.
This gas does the following: Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand.
This is the area inside the bolt carrier where the bolt lives.
Gas expanding here forces the bolt carrier back AND the bolt forward.
Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt.
For a short moment in time, these forces are about equal.
Ideally, this is while the bolt lugs are unlocking and before the extractor starts pulling on the case.
The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring. All of these effect timing, that's why we have different weights of buffers; standard, heavy (H), H2, H3 etc.
The next thing the carrier encounters are the cam surfaces against the cam pin.
Of course we know that the cam pin goes through the bolt.
Rearward movement of the bolt carrier causes the bolt to rotate.
(pay attention here, this is the meaty part)
Here is where timing comes into play.
Let's make a couple assumptions here before we continue.
Trust me that pressures in the case hold the case into the chamber, even though the chamber is slightly tapered.
Also trust me that when you release all the pressure out the front of the barrel that the cartridge case will spring back down to size so it's no longer a tight fit in the chamber as it was with the gas pressure present.
So here's where timing comes into play.
We want the bullet to be out of the front of the barrel AND the pressure to have subsided enough that the case shrinks down BEFORE the bolt lugs are unlocked because when the pressure is high, the case WILL try to stay in the chamber.
Now is the perfect time to point out that one sure sign of high pressures are the fact that the case extrudes into the ejector plunger hole on the bolt and the resulting pressure unlocks the bolt while pressures are still high.
This extruded brass gets wiped off the end of the case head, leaving a shiney spot and the brass usually makes it's way under the extractor, later causing extraction problems we will get to in a little bit.
Here is a graphic illustrating what happens when pressures are too high and the gas system is getting too much gas:

http://www.ar15barrels.com/tech/overpressure.jpg

Now back to extraction, normal/correct version: Pressure subsides, bolt unlocks, carrier momentum continues rearward, pulling the fired (and contracted) cartridge case from the chamber.
As the cartridge case reaches the ejection port, the case pivots on the extractor hook from pressure of the ejector until it is sent flying free of the rifle.
The bolt carrier continues backward while re-cocking the hammer until operating spring pressure or the buffer stops it.
Operating spring (buffer spring) returns the bolt carrier forward where it strips another round from the magazine up the feedramps and into the chamber.
Cartridge stops in the chamber, bolt continues forward, causing the extractor to snap over the rim of the cartridge case.
Bolt finally stops against the case head, but the carrier continues forward.
The cam surfaces in the carrier now cause the bolt to lock into battery again.
Now we are back where we started before we ever pulled the trigger.
Now for extraction, the WRONG ways.
First, too much gas(most common):
The bullet has not left the barrel yet, but it's past the gas port.
Too much high pressure gas is rushing into the carrier, causing it to move rearward faster then desired and unlock the bolt from the extension.
Pressures are still high so the cartridge case is NOT ready to be extracted yet.
The carrier's momentum continues to pull backward, but the pressures in the case actually hold in in the chamber.
This causes a hiccup in the carrier's momentum.
Depending on the severity of the timing, several things can occur:
#1 The (weak) extractor spring allows the extractor to jump over the rim of the cartridge and the bolt carrier continues rearward, grabbing the next round and causing the classic "fired case in chamber, live round behind it" FTE.
The brass shavings under the extractor usually contribute to this one as well.
#2 The extractor does NOT slip off the case, but keeps pulling.
The extractor is strong enough to RIP the rim right off the case.
Same result as above, but MORE brass shavings everywhere from ripping case rims off.
#3 The extractor does NOT slip off the case, but keeps pulling.
During this pulling, the bullet has JUST left the bore, pressures recede and the case shrinks down, allowing extraction.
The rest of the cycle goes as normal, but you have strong pull marks on the case.
Recoil will be higher than normal when the carrier is allowed to travel to the end of the buffer tube and bottom out swiftly against the end of the buffer tube.
In normal operation, the buffer just kisses the end of the tube.
Somewhere between here and the next section, we have proper operation.
Lastly, not enough gas(less common):
The bullet is out of the bore, pressure is subsided, case is extracted and on it's way to ejection.
Depending on the severity of the lack of gas, the bolt carrier may not even get the case out of the chamber before the operating spring returns it forward.
Adding more gas, the case just barely gets out of the ejection port, but the bolt grabs it on it's way forward, classic stovepipe.
Add more gas and the cartridge clears the action, but the bolt does NOT get far enough back to strip a round from the mag.
This is classic short stroking.
You have a single shot action which extracts and ejects, then closes on an empty chamber after you fire it.
In this condition, the bolt will also ride over an empty magazine and close on an empty chamber.
Add some more gas and you will reach the point where it feeds from the magazine and ALMOST works properly, but it still closes over an empty mag.
This is two things, first, poor mag springs are not pushing the follower up fast enough to catch the bolt and second, the bolt is not quite making it back far enough to catch on the magazine follower.
Add just a little more gas and you are back to proper function.
Now, take note, that a lack of gas in a rifle that was functioning fine before can be from several things:
Gas key screws poorly staked and they loosened up, allowing some gas to escape instead of doing it's job INSIDE the bolt carrier.
Gas ring gaps are aligned, gas rings missing or broken, allowing extra gas to flow past them.
Gas block/front sight base is loose, allowing gas to escape before it even gets down the gas tube.
Gas tube "mushroom" is severly worn, probably because it was not properly aligned with the gas key and gas is escaping there.
Here is a picture of a worn out gas tube mushroom:

http://www.ar15barrels.com/tech/gas-tube-wear.jpg
Please note that failure to extract/eject is a symptom of EITHER too much or too little function of the action.
FTE alone is not enough information to decide what to change to fix the problem.
You need to look for other signs such as the excessive recoil and case rim pulling of too much gas or the short stroking of too little gas.
Unfortunately, many guys who don't understand the magic above always ASSUME that they have too little gas.
What do they do? They open up the gas port.
Following the examples above, you can see this only makes the problem worse.
Whew, that was longer than I expected to write.
Hope it all sinks in and you can benefit from it.
Edited 2-3-06 to add more about gas port pressures:
We often hear about mid-length being smoother cycling or pistol being harsher cycling than the typical carbine length gas systems.
Below is a plot of a 223 load.
I have noted the locations of the various gas ports in blue.
You can plainly see what pressures are introduced into the gas systems when the bullet JUST passes the gas port.
This is the reason for the way the various gas system lengths function differently.
The longer the gas system from the chamber, the smoother the gun runs, allowing for the fastest double-taps without the gun jumping off target.
Projectile travel at the bottom assumes that the bullet starts out about 1.5" from the breech, so add 1.5" if you want to compare velocities at different lengths.

http://i245.photobucket.com/albums/gg51/starmetal47/223plot.jpg
There has been a lot of discussion lately about dwell time and how it relates to the function of certain barrels, particularly 18" rifle-gassed, 18" mid-length and 14.5" mid-lengths.
I put together a new graphic to illustrate DWELL TIME.
To use this graphic, find the gas port location above the pressure trace (Orange marks) and then locate the barrel lengths below the pressure trace (Blue marks).
The DWELL TIME is the time between these two marks (indicated on the X axis of the graphic as well as in the data below the graphic.
Optimum dwell time is right around 0.200 ms when you use the two most common gas system/barrel length configurations of 20" rifles and 14.5" carbines.
http://i245.photobucket.com/albums/gg51/starmetal47/pressure-time.jpg

Joe

[StarMetal]

Wrong? Different chambers? Mike, come on. Your pullin my leg here right? :grin:

Back in olden times, the 55 grain bullet of the ball ammunition was loaded to 60,000 CUP while the 223 was loaded to 50,000 CUP.

Both cartridges produce 3200 fps with that weight ball and length barrel.

And the actions of the M16 carboned up for a reason.

Bass,

There are more then one chamber style in AR15's. The NATO and the Wylde just to name two. The chamber in commercial bolt guns is much different then the NATO chamber. The bullet for 5.56 may, I said may, be loaded out further then standard 223 ammo. The NATO chamber is definitely more generous then the commercial chamber and has much much more freebore so the hottly loaded NATO is not problem in this chamber. You are not correct about firing NATO in a commercial chamber, nor correct that there are not two or more different chamber dimensions.

Joe

[mike in co]

Wrong? Different chambers? Mike, come on. Your pullin my leg here right? :grin:

Back in olden times, the 55 grain bullet of the ball ammunition was loaded to 60,000 CUP while the 223 was loaded to 50,000 CUP.

Both cartridges produce 3200 fps with that weight ball and length barrel.

And the actions of the M16 carboned up for a reason.


no i'm not joking.
it's well known by..."knowledgeable" ar shooters. posted on most manufactures sites. in all probability why the gun failed.

223 ammo and usgi ammo are actually tested to different standards.

most "usgi" style ar's come with 5.56 chambers and often chrome lined bores. target or varmit ar's typically come with a 223 "match" chamber...the wylde( after bill w...one of the originators of custom target ar's) is probably the most common.

mike in co

[mike in co]

joe, nice write up, but the wrong powder. use blc2 as it is closest to wc846/wc844. lower pressure for velocity.
lower pressure at gas port for a 20"

i still say your gonna have to prove it to me ...that there is more than trace pressure at the "key" before the bullet has left the bbl.

i recall a disscusion in the past where transducers said i was right.....but i'll listen

its still aprox .25ms from port exposure to bullet exit.

thanks
mike

[felix]

Do not the military 223 rounds have glued in bullets? That would change things considerably during ignition, as well as the sloppier throating. The 308 military "standard" rounds are quite mild because of the many older make-shift bolt guns using them. Anyway, I was given some 223 match cases headstamped BlackHills (they typically use Olin slugs made to order). These cases were prepped for my BR 222 Ackley, and while doing so I noticed how uniform that brass was. Each neck was extremely consistent and definitely meant for a gun with a 246 neck using 224 bullets. A military throat in contrast would be no less than 250, and more like 255. Joe, what does your AR measure? Mike, what's yours? In other words, if John shot those match rounds in his 223, I bet they would not have shown such a wild pressure, unless the bullets were seated too far out. Even then, I don't think the match ammo would have glued in bullets unless told otherwise. ... felix

[StarMetal]

Do not the military 223 rounds have glued in bullets? That would change things considerably during ignition, as well as the sloppier throating. The 308 military "standard" rounds are quite mild because of the many older make-shift bolt guns using them. Anyway, I was given some 223 match cases headstamped BlackHills (they typically use Olin slugs made to order). These cases were prepped for my BR 222 Ackley, and while doing so I noticed how uniform that brass was. Each neck was extremely consistent and definitely meant for a gun with a 246 neck using 224 bullets. A military throat in contrast would be no less than 250, and more like 255. Joe, what does your AR measure? Mike, what's yours? In other words, if John shot those match rounds in his 223, I bet they would not have shown such a wild pressure, unless the bullets were seated too far out. Even then, I don't think the match ammo would have glued in bullets unless told otherwise. ... felix

Felix,

Off the top of my head I forget the exact figure, but your posted numbers sound about right. Lots of freebore in those NATO 5.56 chambers.

I leaning towards the bullets aren't sealed but the primers are.

Joe

[1Shirt]

To me the whole issue is a factor of who cares, as if there is any difference, it is minute, and of no consequence down range.
1Shirt!:coffee:

[HORNET]

Joe, regarding post#44: You getting bored or something? That's a pretty extensive write-up and a lot more detailed than I expected. Nice work.
Way back yesterday in post#5, you were going to run some loads through your AR with the gas system on and with it off. Same chamber, barrel, and load so nothing changed except for tapping off a minuscule amount of gas to provide a solid comparison. I do believe that doing that and posting the results is about the only way to give some solid backing to the theorizing.

Probably still won't make some people happy, though......:( [smilie=l:

[dominicfortune00]

Uncle R.

In a 1911 the barrel never moves but two ways and that is up and down. The barrel is carried back and forth by the slide.

Joe

Actually Joe,

The barrel in a 1911 moves in an arc, being contrained by the link pinned between the barrel and the frame.

Since this link never changes length, to move the barrel has to move in an arc, backwards and downwards at the same time.

[StarMetal]

Actually Joe,

The barrel in a 1911 moves in an arc, being contrained by the link pinned between the barrel and the frame.

Since this link never changes length, to move the barrel has to move in an arc, backwards and downwards at the same time.

Reckon you're not seeing it my way. The barrel is locked into the slide by the locking lugs. The barrel, per se, doesn't move on it's own...so I'm partially wrong about it moving the way I said. It gets yanked down out of the slide in an arc.

Joe

[mike in co]

Do not the military 223 rounds have glued in bullets? That would change things considerably during ignition, as well as the sloppier throating. The 308 military "standard" rounds are quite mild because of the many older make-shift bolt guns using them. Anyway, I was given some 223 match cases headstamped BlackHills (they typically use Olin slugs made to order). These cases were prepped for my BR 222 Ackley, and while doing so I noticed how uniform that brass was. Each neck was extremely consistent and definitely meant for a gun with a 246 neck using 224 bullets. A military throat in contrast would be no less than 250, and more like 255. Joe, what does your AR measure? Mike, what's yours? In other words, if John shot those match rounds in his 223, I bet they would not have shown such a wild pressure, unless the bullets were seated too far out. Even then, I don't think the match ammo would have glued in bullets unless told otherwise. ... felix


what joe said.

my br match chamber has a .250 neck.....i have six or so more chambers that i have not measured, but i do use i ptg wylde reamer for chambers i do myself...will have to look up the specs on it..( did not find spec sheet..but it measures aprox .255)

mike in co

[felix]

Mike, the 253 neck appears to be Sammi 223 minimum, so your guns straddle that demarcation. For example, your 250 would be fine for normal match brass, but still fat for 246 ammo from BH. Maybe the auto guns need that extra margin at 250 for 246 ammo to feed and lock positively. I don't know. Maybe you can ask your buddies on the AR board(s) what their necks are using some "standard" match brass. ... felix

[StarMetal]

Guys, this is a good read also especially the different OAL cartridge sizes for the various chambers.

http://www.zediker.com/articles/AR_chambers.pdf

Joe

[mike in co]

Mike, the 253 neck appears to be Sammi 223 minimum, so your guns straddle that demarcation. For example, your 250 would be fine for normal match brass, but still fat for 246 ammo from BH. Maybe the auto guns need that extra margin at 250 for 246 ammo to feed and lock positively. I don't know. Maybe you can ask your buddies on the AR board(s) what their necks are using some "standard" match brass. ... felix

i believe 223 chamber min is 0.254 +.002..0.255 +.002 at the shoulder
223 ammo is ........................0.253 +.002.....(nra drawing)

hmmmmm i thought my 250 neck was saami min....guess not.
i do turn my necks for that gun.

mike in co

[rwt101]

I wish I had 1/10th the knowledge of some of the people on this forum. It is really great.
Bob T

[Larry Gibson]

Bass Ackward

Back in olden times, the 55 grain bullet of the ball ammunition was loaded to 60,000 CUP while the 223 was loaded to 50,000 CUP.

Just want to point out that your use of CUP for both isn't quite correct. SAAMI standard MAP for the .223 is 55K psi (piezo) or 52K CUP. SAAMI standard most often these days is taken with a conformal piezoelectric transducer. However the U.S. began using gas piezoelectric transducers located at the case mouth with the advent of the 7.62 NATO and continued with the 5.56 NATO. Thus comparison of the 3 seperate methods just by raw numbers is misleading. CIP's standard is 62K psi MAP for both the .223 and the 5.56 taken with a piezoelectric conformal transducer.

I have measured the psi of both US commercial (Winchester, Federal and Remington) and vaious lots of M193 ball in a 21" barrel with a tight SAAMI spec chamber. The commercial ammuntion all gave lower MAP psi (52-55K psi) than the 5.56 M193 (55-60K psi). The velocity of the commercial 55 gr ammo was also less than the 5.56 M193. The commercial .223 averaged about 40-60 fps less. Most of the 5.56 M193 averaged right at 3183 - 3271 fps. I did have one lot of Winchester white box M193 of recent vintage that gave excessive pressure 65,600 psi) and excessive velocity (3351 fps) in both the SAAMI spec chamber and a 5.56 NATO spec chamber.

Just FYI and suspect you were generalizing. I for one am like 1Shirt; the difference between them really doesn't matter. I've put enough of each down range to know 5.56 NATO in a SAAMI spec chamber won't hurt anything. However, in some match type chambers one could run into problem especially with the likes of that Winchester white box 5.56 M193. It sure was "smokin":-)

Larry Gibson

[arcticbreeze]

Just wondering if a bolt action rifle has more velocity than a semi-auto? That is using the same ammo and same length barrel. I thought that the bolt action would be higher because in the semi auto it has to cycle the action. Any thoughts on it?
Bob T

To the OP, the short answer is the difference if any is probably insignificant. The long answer is "read the previous 3 pages".

[rwt101]

I've been reading. It has been very interesting to say the least.
Bob T

[nicholst55]

Nothing in a gas auto moves until well after the bullet has left the barrel - by design. The very tiny volume of gas that is bled off into the piston or other system is so small that it accounts for less than the variation in pressure between any two shots.

So any difference in velocity between the two gun types is due to some other factor, of which there are many.

That may not be entirely true. The bolt begins to unlock on an M4 Carbine (14.5" barrel) before the bullet exits the muzzle. They are notoriously hard on bolts, gas rings, and brass for that very reason. This also contributes to their extremely high cyclic rate of fire in full auto configuration. That's fairly well known, and one reason for the popularity for mid-length gas systems on AR carbines. On the M1A and M14, the gas piston begins to move, and normally contact the op rod, before the bullet exits.

Still, I would agree with Rocky's conclusion that the small difference in gas pressure has negligible effect on velocity between gas guns and bolt guns.

FWIW, I see that Springfield Armory, Inc. has trademarked the name 'Gas Gun.' Guess we need to write it as Gas Gun(tm) from now on. :???:

[KCSO]

With the 45 acp there is more difference between guns than there is between a semi auto and a single shot and a revolver. We tested 9 45acp guns with the same nominal barrel length and of the 4 1911's in the group there was more difference than between the revolvers and the contender.

[AZ-Stew]

Guys, this is a good read also especially the different OAL cartridge sizes for the various chambers.

http://www.zediker.com/articles/AR_chambers.pdf

Joe

This not a shot at you, Joe, just more info for the discussion. Zediker's article clearly answers some of the questions that have been danced around by a number of folks who have posted to this thread, but...

Zediker overlooks one thing in his description of seating depths for the five different chamber specs in his article. That is bullet length. I have called several of the bullet manufacturers hoping to find one who could tell me what their standard was for the distance from the meplat to the junction of the ogive and shank (OSJ (Ogive/Shank Junction)) for a given bullet. The answer was the same from all: We don't spec that distance. The location of the OSJ with respect to the case head is far more important than the COL because the rifling lands will contact the bullet ogive a couple of thousandths above the OSJ when the bullet begins to move upon firing.

This creates a problem for handloaders. You can't measure overall loaded cartridge length and know what the distance is from the case head to the OSJ because the distance between the meplat and the OSJ differs from bullet to bullet within a box of the same bullets. The only way to measure the distance from the case head to the OSJ is with a gage that contacts the bullet as close as possible to the OSJ. I made one on my lathe using a drill size just smaller than the bullet shank. It rests on the ogive just a hair above the shank. I can make comparative measurements for any bullet of that diameter using the gage. One jaw of my caliper is on the case head, the other is beyond the meplat on the end of the gage opposite where the bullet enters.

With the gage, I can make up a dummy round with any new bullet and compare it's OSJ location to that of a dummy round I've made that had the bullet seated to just touch the lands. My Forster competition seating die with the micrometer seating depth adjustment can then be used to quickly adjust the seating depth for the new bullet.

Measure the COL of some of your completed rounds sometime. Chances are you'll find they vary by several thousandths. Consider the implications.

Regards,

Stew

[felix]

That is why it is desirable to find the proper powder speed while seating OFF the lands. ... felix

[AZ-Stew]

Right, Felix, but you still have to know where "off the lands" is. My point is that you can't locate that point using COL because bullet length varies. Using COL, one seated bullet may have the OSJ a few thou off the lands, the next may be on or into the lands.

Regards,

Stew

[mike in co]

This not a shot at you, Joe, just more info for the discussion. Zediker's article clearly answers some of the questions that have been danced around by a number of folks who have posted to this thread, but...

Zediker overlooks one thing in his description of seating depths for the five different chamber specs in his article. That is bullet length. I have called several of the bullet manufacturers hoping to find one who could tell me what their standard was for the distance from the meplat to the junction of the ogive and shank (OSJ (Ogive/Shank Junction)) for a given bullet. The answer was the same from all: We don't spec that distance. The location of the OSJ with respect to the case head is far more important than the COL because the rifling lands will contact the bullet ogive a couple of thousandths above the OSJ when the bullet begins to move upon firing.

This creates a problem for handloaders. You can't measure overall loaded cartridge length and know what the distance is from the case head to the OSJ because the distance between the meplat and the OSJ differs from bullet to bullet within a box of the same bullets. The only way to measure the distance from the case head to the OSJ is with a gage that contacts the bullet as close as possible to the OSJ. I made one on my lathe using a drill size just smaller than the bullet shank. It rests on the ogive just a hair above the shank. I can make comparative measurements for any bullet of that diameter using the gage. One jaw of my caliper is on the case head, the other is beyond the meplat on the end of the gage opposite where the bullet enters.

With the gage, I can make up a dummy round with any new bullet and compare it's OSJ location to that of a dummy round I've made that had the bullet seated to just touch the lands. My Forster competition seating die with the micrometer seating depth adjustment can then be used to quickly adjust the seating depth for the new bullet.

Measure the COL of some of your completed rounds sometime. Chances are you'll find they vary by several thousandths. Consider the implications.

Regards,

Stew
two things stew,
1) stoney point(now hornady ibelieve) among others make that tool.
2) unless the seating die contacts the tip of the bullet(few do in the condom world of rifles), the lenght to the lands is the same with most dies...cause they touch the bullet at some dia,and the same spot on all the bullets. as far as COAL....it can be as much as 0.020 off from round to round(and still have the same ogive aol). jacketed oal should vary...cause the bullet length varies. not sure about the newer plastic tipped bullets.

mike in co

[AZ-Stew]

Mike,

My choices were: 1.) Pay Stoney Point, 2.) make my own for a few minutes work and a few cents worth of scrap material. I chose #2.

Assume for a moment that you establish your COL using the longest bullet out of a new box of heavy target bullets (remember, they're all different lengths), and you got the longest one by random selection, not knowing the bullet lengths vary. You load 50 rounds, shoot them and get good results, but maximum velocity, indicating that you're pushing the pressure limit. Next time at the bench you load some rounds using a different weight bullet, say, a light weight bullet for varminting, and have to adjust your seating die for that bullet. After your varminting safari you return to the bench to load rounds for your next F-Class match using the heavy target bullets. In order to set your dies for that bullet, you measure COL again, but this time, just by chance, you chose the shortest bullet from the box. Unbeknownst to you, you've just moved the OSJ forward to the point where all loaded rounds in this batch have the ogive jammed firmly into the lands. Where does your pressure go? That was the point. COL is not a good indicator of the relationship between the OSJ and the case head.

Regards,

Stew

[mike in co]

Mike,

My choices were: 1.) Pay Stoney Point, 2.) make my own for a few minutes work and a few cents worth of scrap material. I chose #2.

Assume for a moment that you establish your COL using the longest bullet out of a new box of heavy target bullets (remember, they're all different lengths), and you got the longest one by random selection, not knowing the bullet lengths vary. You load 50 rounds, shoot them and get good results, but maximum velocity, indicating that you're pushing the pressure limit. Next time at the bench you load some rounds using a different weight bullet, say, a light weight bullet for varminting, and have to adjust your seating die for that bullet. After your varminting safari you return to the bench to load rounds for your next F-Class match using the heavy target bullets. In order to set your dies for that bullet, you measure COL again, but this time, just by chance, you chose the shortest bullet from the box. Unbeknownst to you, you've just moved the OSJ forward to the point where all loaded rounds in this batch have the ogive jammed firmly into the lands. Where does your pressure go? That was the point. COL is not a good indicator of the relationship between the OSJ and the case head.


Regards,

Stew

you would be foolish to measure col if you already know the seating depth. return the seater to the known position. use mirometer tops, measure the stem height....make up a dummy round ..use it to reset the seater........
what you have described is a "reloader", i consider myself an ammo crafter.
i have no idea why anyone would do what you describe....but thats just me.

again , anyone that does not understand that bullet lengths vary is not an ammo crafter...just a reloader...and can expect poor results for his work.


mike in co

[StarMetal]

Well I'm at a lost. I put 10 round over the chrono with the gas system on and 10 with the gas system off in my AR15. The load was 62 gr IMI FMJ over top some 748. Not a hot load.

On the velocity was 2798
Off the velocity was 2773

:groner:[smilie=b:

Don't ask me, I don't know. Ammo been loaded years about by me and I'm not one to change a powder weight on my 5.56. Might be the chrono messing up.

I'll try the 7.62x39 out of the Yugo next time out.

Joe

[yondering]

Well I'm at a lost. I put 10 round over the chrono with the gas system on and 10 with the gas system off in my AR15. The load was 62 gr IMI FMJ over top some 748. Not a hot load.

On the velocity was 2798
Off the velocity was 2773

:groner:[smilie=b:

Don't ask me, I don't know. Ammo been loaded years about by me and I'm not one to change a powder weight on my 5.56. Might be the chrono messing up.

I'll try the 7.62x39 out of the Yugo next time out.

Joe

Did you shoot the 10 rounds "on" first? Bore fouling does build up enough to change the velocity by the amount you listed, with some powders more than others. I've found up to 50 fps drop in average velocity over a 50 round string in my 22 Hornet with jacketed bullets. For a fair comparison, shoot one "on", one "off", ect, until you've fired 10 of each.

[StarMetal]

Did you shoot the 10 rounds "on" first? Bore fouling does build up enough to change the velocity by the amount you listed, with some powders more than others. I've found up to 50 fps drop in average velocity over a 50 round string in my 22 Hornet with jacketed bullets. For a fair comparison, shoot one "on", one "off", ect, until you've fired 10 of each.

I shot the 10 off first. I figured if I fouled my gas port hole with carbon it would clear it out in the on position.

Joe

[Larry Gibson]

Well I'm at a lost. I put 10 round over the chrono with the gas system on and 10 with the gas system off in my AR15. The load was 62 gr IMI FMJ over top some 748. Not a hot load.

On the velocity was 2798
Off the velocity was 2773

:groner:[smilie=b:

Don't ask me, I don't know. Ammo been loaded years about by me and I'm not one to change a powder weight on my 5.56. Might be the chrono messing up.

I'll try the 7.62x39 out of the Yugo next time out.

Joe


Nothing out of whack there. The 15 fps variation is within the variattion of mean averages of different strings of the same lot. While I have not tested the 5.56 in an AR where the gas can be shut off I have ran a similar comparitive test with other cartridges (7.62x39, 7.62x54R, 7.62 NATO, 30-06 and 8x57) in different semi/full auto's where the gas can be shut off. The results were pretty much the same; the mean average velocity with the gas on or off fell within the ES of all the lot tested. The ES of the mean average velocities with the gas on or off fell with in accepted variation.

In other words Joe, your first post on this thread/subject is essentially correct; the bullet has gotten all the velocity it will get before the action begins to use the residual gas force to function.

Larry Gibson

[Ed Barrett]

I would go out and chrony some loads both ways but it too darn cold here!

[StarMetal]

Nothing out of whack there. The 15 fps variation is within the variattion of mean averages of different strings of the same lot. While I have not tested the 5.56 in an AR where the gas can be shut off I have ran a similar comparitive test with other cartridges (7.62x39, 7.62x54R, 7.62 NATO, 30-06 and 8x57) in different semi/full auto's where the gas can be shut off. The results were pretty much the same; the mean average velocity with the gas on or off fell within the ES of all the lot tested. The ES of the mean average velocities with the gas on or off fell with in accepted variation.

In other words Joe, your first post on this thread/subject is essentially correct; the bullet has gotten all the velocity it will get before the action begins to use the residual gas force to function.

Larry Gibson

Sure surprised me Larry. I'll still try the MAS 49/56 and one of the 7.62x39's just for the heck of it. Heck it gives me a reason to shoot doesn't it? :Fire:

Joe

[Larry Gibson]

Starmetal

We need a reason?

LOLs

Larry Gibson

[AZ-Stew]

Mike,

You've described some good ways to avoid the problem I mentioned, but that's not why I brought it up. I specifically mentioned not using COL to determine OSJ to case head distance. Look in the handloading manuals. They almost always specify COL as a determining factor for seating depth. Since most of the manuals are written by bullet manufacturers, they should know better. I'd be willing to bet that 95 plus percent of the members here don't know that the distance between OSJ and meplat varies within a bullet lot. In addition, not all handloaders use a micrometer seating die. They have to have another method of determining seating depth. I'll wager most don't create dummy seating depth rounds for all bullets. Doesn't make them bad folks, just uninformed.

I brought up the subject because so many handloading sources use COL as a determining factor. A lot of handloaders rely on such info. Remember, it's in the "published data" (handloading manuals) that you and I and others constantly refer newbees to when they ask basic questions. They're ignoring some important facts. We both know it. I'm just making sure that the readers of this thread know it, too.

Regards,

Stew

[twidget]

Just wondering if a bolt action rifle has more velocity than a semi-auto? That is using the same ammo and same length barrel. I thought that the bolt action would be higher because in the semi auto it has to cycle the action. Any thoughts on it?
Bob T

The NRA answered that question many years ago. I'm doing this from memory so I may not get all the details right, but the overall result is basically correct. They used an M14 and fired strings of shots with the gas system open (semi-auto) and closed (single shot). I don't remember how many shots they fired, but it was enough to give a statistically valid result. Yes, there was a difference, but it was small - around 15 fps more with the gas system closed. That's more than the shot-to-shot variation and pretty much invisible unless you are looking for it.