Question About Velocity and Pressure

[Jeff82]

I've always assumed that velocity for a given weight of bullet is a proxy for pressure. I also have always thought that when we match bullet hardness with different velocities that this was really relating the pressure to BHN relationship in a easy to apply way. Then I remembered that in the early days of BP cartridges velocities were kept to under 1,200 fps (ignoring paper patching) and the larger and heavier bullets were used to design cartridges with more power while keeping velocity down.

Lately, I got this idea that I could avoid gas checking my 357 cartridges by just using a less powerful loading; simple so far. Then I got to thinking that it would be a good idea to develop a less powerful cartridge that duplicates the trajectory of my full power 357 loads. This is for my Marlin carbine. So, if my regular full-power load has a 158-grain bullet moving at 1,400 fps, I'd make cartridge with a 125 grain bullet that also travels at 1,400 fps, and then tweak the load until I match the trajectory.

Then I started thinking about it and am wondering if it's true that velocity is just a proxy for pressure. Because if it is, then I don't think they would have designed BP cartridges with the 1,200 fps limit that they did in the old days.


Can anyone clue me in on this?

Thanks,


--Jeff

[gloob]

Personally I think velocity is the more important factor over pressure, anyway. For instance, I can shoot full tilt h110 loads out of a revolver using plain base bullets with a velocity in the 1300 fps range. But out of a carbine, common knowledge dictates that over 1500-1600 fps needs a gas check. Pressure is the same in either case, but out of the rifle, I (assume) the bullet will foul and not be accurate. The only difference is velocity. Hmm, maybe the longer barrel is an important factor, too. By the time the bullet makes it to the end of a 20" barrel, it is probably a bit hotter than when the bullet is 6" down the barrel.

If pressure was where it was at, you could easily load cast bullets in centerfire rifles with slower powders. But the best powders tend to be FASTER powders than for jacketed bullet loads, even though there is PLENTY of case capacity leftover.

So I don't care if velocity is a proxy for pressure or not. The velocity is more important than the pressure, anyway. I don't really care what the pressure is, as long as it's under SAAMI max. Pressure is not something most folks can measure, anyway.

[tazman]

Pressure doesn't necessarily translate to velocity unless you are only using one powder.
By using a slower burning powder, you spread the pressure out over a longer distance down the barrel, giving more velocity for the same or less pressure.

[Newboy]

Oehler uses that idea to "calibrate" their strain gauge system. But it does have reasonable limits. I believe velocity would be more in proportion to the area under the pressure-time curve.

[mongoose33]

I used to think that too but I've backed away from that idea. There are several variables that go into pressure, including how slippery the bullet is and the type of powder (progressive burning versus not).

I like to chronograph everything I work up just to see if something's out of whack, but as a direct proxy for pressure? I think it can be dangerous to assume that.

[leftiye]

If that were true then cast boolits and jacketed boolits of the same load in the same gun would achieve the same velocity. As you know, the cast boolit will shoot faster AND at a lower pressure (mongoose33's slippery boolit). Newboy's area under the pressure curve determines pressure/time thrusting on the bullet, other factors impact this to add or lessen the actual acceleration. Chronographs only poorly indicate pressure. Pressure is what you need to care about. Pressure, not velocity will blow up your gun. In many cartridges of ancient (1873) origin, loading them is all about loads that achieve higher velocities without damaging older guns. Different powders with different pressure rises and different pressure curves make a major difference in low pressure achievement of high velocity.

[Ballistics in Scotland]

In the absence of friction the acceleration of a body is directly proportional to the force acting upon it, and inversely proportional to its mass. For a bullet of a given diameter the area of its base is constant, and the thrust is proportional to pressure. The light bullet therefore requires less pressure to accelerate at a given rate.

If two bullets have the same velocity after travelling to the muzzle of the same barrel in the same period of time, they have experienced the same average acceleration. That is not to say it has been the same all the time. An excessively fast powder may have built up rapidly to a extremely high pressure (with its risk of a burst, difficult extraction, excessive gas escape past the bullet, metallic fouling etc.) which has fallen off to very low levels later. An excessively slow powder, or one with a retardant coating the specific application doesn't burn through fast enough, may have started slow and ended up with a high terminal velocity, producing noise, blast, and a waste of powder energy as either flash or unburnt grains.

Bullet material may make some difference in friction, first by forcement (the need to squeeze it into the rifling) and then on the front edges of the rifling grooves. The faster the twist of the rifling, the greater this friction and the heat it generates in the bullet. As long as the bore is of constant cross-section, rotation is probably the major cause of friction.

[44man]

BP velocities were not kept low on purpose, the powder just has limits. By adding more powder then a case demands will actually reduce velocity with unburned powder exiting the muzzle. That is why they made larger cases. Too much powder means more compression that makes it harder to burn.
Too much compressed BP will increase pressure and recoil because it just adds to boolit weight. Large cases means more powder with less compression and it burns better. Still tough to go much over 1200 fps.
Pyrodex is the worst if compressed too much, velocity will go down, recoil and pressure goes up and hunks of Pyrodex will fly out the muzzle to burn like flares in the grass. 777 should not be compressed at all and a boolit should just be against it.
The pellets used now have holes through them and a very hot primer is used.
Many of us with the 45-70, using the right amount of BP can get perfect ignition with LP primers and a newspaper disk over the flash hole to keep powder out of it. Do not enlarge flash holes, you can drive a hammer to full cock and take a chance at gas leakage. It is the same as a large hole nipple on a ML. It can cock a hammer and blow cap particles into your face.

[Larry Gibson]

Jeff82

The only correlation between velocity and pressure will be if the same cartridge, bullet, powder, cases, primers and firearm are used with only the powder charge being varied. Other than that attempting a correlation can be misleading at best and very disastrous at worse.



Newboy

"Oehler uses that idea to "calibrate" their strain gauge system."

I would be interested in a further explanation how Oehler "calibrates" the strain gauge system?

Larry Gibson

[Pilgrim]

Larry - Give Oehler a call and talk to him. Very nice guy, easy to talk to, and not afraid to tell you how he does stuff. He doesn't have any fear you're gonna take business away from him so he'll give you the info you want. I seriously question using velocity to calibrate strain gauges. Everything being equal, pressure will vary barrel to barrel, everything being equal. OTH - if he is using the same barrel, same load (including powder and primer lots), same bullet (also from the same lot), same or identical cases and then is comparing pressure traces, I'll
not quarrel over that use.

Post #7 above lays out the simple physics of the acceleration & time factors, but does not point out that exterior ballistics is at play in your goal. Different weight bullets/boolits, given the same form factor, same velocity, etc. will not give you the same trajectory due to different ballistic coefficients, and sectional density. The lighter bullet/boolit will slow down faster than the heavier projectile every time so your goal is very likely not achievable.

FWIW...Pilgrim

[Ballistics in Scotland]

Some of the very large black powder express cases got very little increase in velocity over the smaller ones. The maximum achievable velocity is pretty constant, but a heavy charge allows it to be achieved with a heavier bullet. Something like useless friction would have been achieved long before a black powder round achieved the proportions of, say, the .264 Winchester Magnum.

Pellets aren't a new idea. In the days of the Snider (a fairly short straight-cased .577) they were found to give insufficient consistency of pressure. But they were used in rockets and in huge grains for huge cannon, with a tapered central hole.

The same principle was used in the first Lee-Metford .303 round, although the rifle had been designed with the idea that the mysterious French smokeless powder would become available, and the sights were calibrated for a correctly guessed hundred ft./sec. or so more than black powder could achieve, but about right for the 2040 ft./sec. of the smokeless load it soon used. Even 1950ft./sec. was pretty miraculous by black powder standards. The interesting thing about the pellet is that the primer necessarily couldn't ignite all the charge at once. It seems as though igniting a consistent part of it is what matters.

The burning rate of black powder remains about proportional to pressure on it, while that of smokeless increases much more than pressure. What this means is that black powder consumption is more closely related to time in the bore. This is why black powder hand firearms can advantageously use a longer barrel than smokeless ones, but with artillery the length of a smokeless barrel would end up being just useless friction.

All this means that velocity is a very unreliable way to measure black powder pressures. The only thing to be said for it is that it isn't nearly as bad as doing so with smokeless. The effect of the burning rate being much more than proportional to pressure, is that anything which tends to run up pressure a little, in fact runs it up quite a lot. Then the burning rate is proportional to that pressure...

[Ballistics in Scotland]

Another point is what happens to the weight of the powder. For one thing it is a mass that moves forward, like the bullet. A fairly good rule for shotguns is to make the shot charge a hundredth of the weight of the gun. In theory the momentum (mass times velocity, not mass times the square of velocity like the projectile energy) imparted to the shot and the gun will be equal. The gun will move rearward at a hundredth the speed the gun moves forward. In fact the powder weight must be added to projectile weight. In a rifle that might add up to 400gr. of lead and 80gr. of black powder. The rifle would move more than in reaction to 30gr. of smokeless.

But that is far from the whole story. On the good side some of the powder hardly moves at all, being left as fouling or stationary gases in the powder chamber of the cartridge. On the other side (and it is much greater) there is a rather strong rocket effect as the released gas suddenly accelerates to a considerably higher velocity than the bullet, and shoots around and past it. (pressure on the base actually accelerates the bullet by two or three percent after it leaves the muzzle, and may be why base irregularities are so much more harmful to accuracy than nose irregularities. Release the neck of an inflated balloon, and it will whiz across the room.

Sheer mass accelerating up the barrel is a minor reason why black powder loads recoil harder for a given bullet velocity, while the major reason is sheer mass taking part in the final rocket blast.

[Jeff82]

Interesting discussion and I'm picking up on a lot of stuff I didn't know. I think some of what I'm asking is misunderstood. I'm not talking about using velocity as a proxy for pressure to work up loads without the aid of a loading manual. I'm asking in reference to the bullet. Most discussions about bullet hardness and the need to gas check are expressed in terms of velocity. I've always assumed that this is because velocity is easier to meaningfully relate to than is pressure.

It's relevant to me because I'm getting really tired of gas-checking my full-power 357 cartridges. I'm thinking that if I held velocity constant using a lighter weight bullet, and charged it based on what the loading manual indicates an equivalent velocity would be to my full-power load, I might be able to get a similar point-of-impact to my full-power cartridge at 100-yards. With all else held constant the lighter weight bullet propelled at the same velocity would create less internal pressure (all else held equal except the bullet weight and powder charge), and might allow me to forego the tedious job of gas-checking my PB-Lee 158-grain RNFP bullets. This only works if pressure is the key driver of the choice to gas-check. If velocity is truly the key, then trying to do this is pointless.

Alternatively, I could save myself the expense of a Lee BP-125-grain RNFP mold, and just work up a lighter charge of my 158-grain RNFPs and try to adjust the charge to the same point of impact at 50-yards and forget about 100-yard performance.

[Larry Gibson]

Pilgrim

I've been talking to Dr. Oehler since 1975 when I got my 1st chronograph from him. I have 2 other Oehler chronographs including the M43 PBL which uses a strain gauge to measure pressures. I have done extensive testing of hundreds of loads in 30+ test barrels the last 8+ years. I have discussed the strain gauge technology extensively with Dr. Oehler and John (who manages things there now) on numerous occasions. The strain gauges come pre-calibrated and that information is input into the M43 and the M82. No other "calibration" is done. The use of reference ammunition is recommended for obtaining the upper and lower "inclusion limits" per SAAMI testing.

Many times individuals, based on internet "experts opinion", attempt to discredit the M43 and other strain gauge pressure testing equipment because they are not "calibrated" by the user. If there is some sort of "calibrating" of the M43 that needs to be done I would like to know about it. I have asked Dr. Oehler about this and the answer is "there is none". The only thing necessary is to enter the "gauge factor" and the correct "Offset Pressure" into the program. Any further "calibrating" is done with reference ammunition if desired. That is not actually "calibrating" either btw.

Thus if someone has any "facts" about calibrating the M43 PBL made by Oehler Research Co. I would certainly be glad to hear about it?

Larry Gibson

[jcren]

These guys are all far more scientific than I, but my .02 is that pressure is at least 90% or the gas check equation. As bullet weight, bearing surface and Gunn bore/rifling all play a part, pressure induced blow-by is the breaking point. So, yes a lighter/slicker bullet could be worked up to full velocity/theoretical trajectory at lower chamber pressure. By the same token, pistol caliber rifles could use a slower burning/more gradual powder to utilize the longer barrel to get higher velocities at lower pressures. Opposite point of fact, to try to get 2700 fps out of my 30-06 with bulls eye powder would be bad.

[Newboy]

Larry, I do not have an Oehler 43. And I do not discredit the technology. And the fact that a strain gauge has been calibrated has almost nothing to do with how it will react on an individual rifle action. The only way to truly calibrate the system is to pressurize the action to a known level, and note the reading on the gauges. This is very difficult to do (see "Rifle Accuracy Facts" by Vaughn for a technique), so approximations are made. It is an approximation to fire a reference load and extrapolate pressure levels. Off topic, and not looking for a peeing contest.

[MT Chambers]

The OP is also talking about cast bullets without gc in the .357, it was the .357 factory ammo very early on that required manufacturers to go with GC cast bullets. For .38 spl. vel. pb may be okay but for .357 I'd opt for gc bullets unless in cowboy action loads.

[Larry Gibson]

Newboy

If you say so. Apparently you're unaware of the test of such gauge systems in comparison with C.U.P measurements, peizo-transducer and conformal transducer measurement and case mouth transducer measurements as conducted by Dr. Oehler?

Not wanting a peeing contest either, just a straight forward discussion so I may learn something. Question; doesn't firing a reference load of known pressure produce the same "approximation" as pressurizing the system by mechanical means? The answer is yes and the mechanical means is no more reliable. Additionally if done with the mechanical means the only thing you end up with is the same offset percentage to recomputed the psi with as the test with reference ammunition gives you. Using an "offset" figure to recalculate the psi is not "calibrating" the instrument as nothing is reset in the instrument.

Besides, the measurement of psi inside a chamber is an "approximation" regardless of the means used. That's why there are various levels of psi testing measurements related to firearms (MAP, MPLM, MPSM, etc.) along with the +/- Inclusion Limits of each. Perhaps a better understanding of "calibrate" might be in order since the use of strain gauge measurement of cartridge pressure is used almost industry wide now in conjunction with peizo-transducers and C.U.P. methods.

It might be of interest to read the information published by SAAMI regards the use of reference ammunition. You'll find the test guns are not calibrated. The transducers are calibrated before installation in the test barrel. After calibration and then installation on the test barrel guess what happens next? Reference ammunition is then fired to get the offset figure. Most of the major ammunition manufacturers then use test firearms with strain gauges attached (the Oehler M82 is pretty much the industry standard) to confirm acceptable levels of pressures and throughout production to maintain quality control. With the C.U.P. system is a tarage table with the copper crusher pellet lots which is based on a crush test of some pellets from the sample lot with a know pressure load. There is not a "calibration" of each test pellet. A conformal transducer requires the lot of brass be used during the test as with calibration.

Every system of calibration of pressure measurement is an approximation. Why? We know in the Bureau of Weights and Measures there is "one inch", one foot, and probably "one yard". There is also almost every other type of physical measurement with which we can compare anything to them. However, is there "one pound per square inch" of pressure on hand for comparison? If you know the location of one please let us know. Fact is we have gauges that says "this is yadda-yadda pounds per square inch". We have calibrated gauges that say that, don't we? I ask what the calibrated gauges were calibrated with........uh, why yes....just another gauge that was "calibrated" that says that's "yadda-yadda psi"!

Fact is all methods of measuring the psi of fired cartridges are approximations made through indirect observation and we have no instrumentation inside the cartridge to measure the pressure. The C.U.P. measures peak psi through the crush of the copper pellet. The gas case mouth, peizo and conformal transducers measure psi through that applied the peizo quartz which creates and electronic signal converted to psi by a computer. The strain gauge measures the "stretch" (strain" placed on the barrel and is converted to psi, also by a computer. So as we see every method of psi measurement of a fired cartridge is an "approximation". Even the very close tolerance pressure test fixtures of the major ammunition manufacturers vary in psi measurement with the same ammunition. That's why they use reference ammunition.

I have read Vaughn BTW. Excellent research but if you check the bibliography under "interior ballistics" the newest research used was published in 1979. There has been a great leap forward since that time in the use of strain gauges and peizo-transducers. None the less a good read, good reference and informative even though dry in some places which is to be expected of an aerospace engineer (rocket scientist).

Larry Gibson

[Newboy]

Larry, this is my last here. Maybe I have not made the point clear. All you have done is circumvent the point. The primary assumption made is that you can take a "reference" load and shoot it in my rifle, and assume it will generate X psi. And that is what you are doing. And I do not condemn that.

FYI, NIST does have inch and pound standards.

[44man]

Back to the original question about PB or a GC in the .357. First understand what a GC is for. It is a skid stop to grip the rifling and allow the boolit to engage rifling without having gas channels open alongside the boolit. The wrong powder can defeat a GC if it peaks full pressure too fast and you can still get leading.
The solution for a PB is to make the boolit tough enough to take the turn without skidding past the base. All of my big bores use a PB.
If you get rid of putty balls in the .357, I see no reason a 158 gr PB can't be used and might work better then a lighter boolit that accelerates faster.
The best way to see what happens is to recover boolits to inspect rifling marks to make sure skid stops on the base band. Marks there should be no larger then land and groove size. A little skid near the nose is OK.
Many say to soften so a boolit re-expands into rifling but that can be a pipe dream. Even a soft boolit that expands to fit the whole forcing cone or tries to escape the gap is only slumped to ruin and will still skid. The worst leading boolits ever made are the dead soft Wad cutters in a .38 with Bullseye, etc. Use a slow powder in the .357.

[Jeff82]

44-man. Good observation. Maybe the easiest thing to do is to continue to work with the 158-grain bullet (bhn = 15), and compare GC with non-GC performance at successively lower velocities to determine the value of gas-checking at different power levels and to find a point or indifference, then work from there.

The same velocity lighter bullet theory may be more problematic than I expected.

[Larry Gibson]

Jeff82

This is for my Marlin carbine. So, if my regular full-power load has a 158-grain bullet moving at 1,400 fps, I'd make cartridge with a 125 grain bullet that also travels at 1,400 fps, and then tweak the load until I match the trajectory.

The problem is to match the trajectory with a given velocity the BCs of the bullets must be the same. With .35 caliber pistol bullets suitable to your Marlin the BC of a 158 gr bullet will always be higher than that of a 125 gr bullet. Thus with both at 1400 fps the 158 gr bullet will always have a different trajectory than the 125 gr bullet. The less recoil of the 125 gr bullet, even at the same 1400 fps velocity, will probably influence the zero also. The pressures of the different loads are not relevant to trajectory if the velocity remains constant at 1400 fps.

Larry Gibson

[rsrocket1]

Larry is right. In the absence of any wind, the ballistic trajectory of a 125g bullet at X muzzle velocity will be very different than a 158g bullet at the same X velocity. Add wind and obviously the lighter bullet will not have the same POI as the heavier one.

Interestingly enough, I ran a couple of QuikLoad/QuickTarget calculations of a 125g bullet at 1400 fps MV and a 158g bullet at 1400 fps MV. The good thing for you Jeff is that at 100 yards, the trajectories of a 1400 fps 125g bullet is very close to a 158g bullet, but of course that is still dependent on the profile of the bullet.

I used the Lee 358-158 RF bullet (round nose flat point BC=0.16) and compared it to the Lee 358-125 RF (BC=0.116). The trajectories are not too far off at 100 yards.
134222
You can see the difference better by looking at the bullet drop at 100 yards. The 158g bullet drops about 10.5" while the 125g bullet drops about 11".

134221
That is pretty close. The big unknown would be how your barrel behaves with different bullet travel times. The 158g bullet will exit the muzzle in about 1.29 mS, while the 125g bullet will exit at about 1.40 mS. That could cause the group to open up quite a bit (or not). This was using 7g Unique for the 158g bullet and 6.3g Unique for the 125g bullet in a 16" barrel, YMMV.

My recommendation is to load the lighter unchecked bullets to whatever gives you the best groups regardless of where on the target the land, keep your sights adjusted for the 158g bullets and compensate (or write down the number of clicks to go back and forth). A better group is more important that the same impact point unless the groups are acceptable. For me, I was able to keep the same IP with both 110g bullets at 1700 fps and 150g bullets at 2460 fps in my .308. The 110g bullet groups were about 2" compared to 1" for the bigger loads, but that was good enough for me.

Good luck

[Jeff82]

Larry and rsrocket1 thanks for the advice and the really cool trajectory comparisons. I think I'm beginning to see that I'm not approaching the problem with realistic expectations. Alternatively, I know that when I shoot lighter loads with my standard 158-grain Lee bullet, the rounds will print at twelve O'clock and up to six-inches high at 100-yards for the charges I was using at one time. I don't know how these lighter charges would perform without gas-checks, but at least I do know that they hit along the vertical axis; presumably due to dwell time in the barrel and barrel jump.