Can Someone Explain This Pressure Slug Up Velocity Comment?

[Silver Jack Hammer]

Allan Jones in Shooting Times Feb 2013 talks about the same boolit “slugging up” differently at the same velocity when fired out of two different cartridges, the 40 Smith and Wesson and the 10mm. He states that the FBI’s Firearms Training Unit found the same thing the Speer laboratory found, that the same boolit gives different terminal numbers when fired at the same velocity out of the two different cartridges. Allan Jones states the Speer lab found that the two cartridges deliver the same velocity at two much different chamber pressures. The test involved the 180 gr .40 S&W at 980 fps. The 40 S&W needed 31,000 to 33,000 psi and the 10mm needed only 24,000 to 26,000 to reach the same velocity. What was discovered was the effect on expansion when bullets “slug up” under pressure. “The bearing surface lengthens at the expense of the nose length due to in-bore friction. The greater the pressure, the more nose shortening occurs. It’s very noticeable in boolits with pure lead cores…” Compared to the lowered pressure 10mm Lite, the 40 S&W pressures shortened the nose more and reduced expanded diameter, a critical parameter in the FBI’s calculations.

[Piedmont]

In slightly different words, the bullet shape is changing and this affects how it performs in gelatin. The shape change is pressure related, not velocity related. So even though the velocity is the same from both cartridges they are not reacting the same when they hit the jello.

[runfiverun]

you are accelerating the one in the 40 sooner.
the same thing happens with cast boolits it's called slump.

[44man]

Shooting nothing but revolvers for extreme accuracy, slump and bump up is not allowed at all. Fit and maintaining boolit integrity to the target is the secret.

[Silver Jack Hammer]

Why do two different cartridges throw the same boolit at the same velocity with two different pressures? How can one cartridge need 31,000 to 33,000 to throw the boolit at 980 while the other cartridge throw the same boolit at the same speed with only 24,000 to 26,000 psi?

As I observe, the only difference is the length of the cartridge case. I am unaware of any principle that shorter cartridges generate higher pressure or require higher pressure to match the same velocity a longer case can achieve with less pressure. If this concept were a general principle, then the same should apply to the .38 / .357, .44 Special / .44 Mag. I've very curious because I see the .45 Long Colt performing very well at low pressure while the .40 S&W is used so prolifically and it is such a high pressure cartridge. Up until now I’ve dismissed comparing the two cartridges because they are so different in so many ways, it would like comparing apples to horses.

And I was unaware of boolit nose deformation in the bore. I shoot a .270 Winchester rifle. I’d think I’d have read something somewhere of boolit nose deformation in the bore if not a pistol than in the rifle, when everybody and their neighbor is writing about the effects expanding boolits upon impact with the target.

[btroj]

Different size pressure vessels and different burn rate powders. Small cases do better with "faster" powders which develop a faster kick in the butt for the bullet, this means more slump. Why not use a slower powder? Not enough room for it. Slower powder means bigger charge.
A 357 can be cruising along and equal what a hot rodded 38 is doing.
The deformation affecting expansion is something the FBI is interested in because they want a sure fire expansion every time. Small differences matter there and any change in nose shape or velocity can make a big difference to them.
Your 270 is shooting a 130 fast enough that it will expand reliably at any sane distance. It shoots a bullet with a wife window for expansion. Handgun bullets expand over a much narrower window.

[Bwana]

The only thing I can see that they did was waste our money finding out something that makes little difference in the long run. When law enforcement hit factor remains about 20 percent they would do better spending that money on marksmenship courses.

I have seen 357 mag jacketed bullets with their bases dished because of soft cores and high pressures. These were recovered from snow banks so there was no other deformation other than the rifling marks.
I also remember an "original" Hawaii 50 series episode where a person was shot with a 45acp. They couldn't figure out how it was done as no one was in the area when he was shot. They finally figured out that because the base of the bullet was deformed, dished, that it had been fired at a much higher pressure than the handgun would have used. Turned out it was a hitman that used a specially built rifle that was fired from some distance away and was rifled just like a 45acp bbl.

[TXGunNut]

10mm was too much for the average Feebie so they backed it down to something they could handle. (Not knocking the Feebies, it's quite a handful.) All this expansion testing is well and good until you need penetration thru a car door, light cover or even heavy clothing. Nothing against the 40, it's a much better duty cartridge than the 10 or most any other handgun cartridge, IMHO. I'm afraid the honest truth is that modern defensive (J-word) handgun bullets at normal velocities only expand about 50 percent of the time in actual shootings. If I can't depend on expansion virtually every time I won't expect it to happen when I want it, my luck just seems to work that way.
If I want a big hole, I start with a big bullet. If I want something to cease all actions I want a big hole thru something important and another hole on the other side to let blood out and air in. What I want from gelation tests is bullet integrity and penetration data. I'm not convinced bullet expansion is all that desirable anyway.

[Silver Jack Hammer]

"I'm not convinced bullet expansion is all that desirable anyway." TXGunNut I agree completely.

Is it a rule that a longer case is going to throw a boolit at a lower pressure and a shorter case going to generate higher pressure at similar velocities?

The reason I am asking this is because I have some nice new Starline Schofield brass and I was going to start developing a Keith boolit load with it in my Colt .45 SAA. Right now I'm running frontier equivalent 230 gr boolit at 830 fps and it seems a little more power can be attained safely.

This whole boolit slugging up in the bore business is new to me. I'm understanding the boolit slugging up in the bore affected boolit diameter measurements after coming to rest in gelatin.

Thanks for the posts guys.

[Piedmont]

Is it a rule that a longer case is going to throw a boolit at a lower pressure and a shorter case going to generate higher pressure at similar velocities?



It is almost a rule. Some of it depends on burn rates but if you use the optimum powder each time you can call it rule. Let me give some examples. Load the .44 special and Mag to the same pressures with 2400 powder and the Mag will always out run the Special by 150 fps or so. How can it? That extra powder is doing something. Compare a 9mm+P+ to a 357 Sig. The +P + I think exceeds it in pressure but lags in velocity. Case capacity. Load a .30-06 to .308 pressures (it isn't from the factory) and it will outrun the .308.

The converse of this is that if you load the larger capacity case to same velocity as the smaller case, again the given being the optimum powder is being used, it will hit that velocity at lower pressure. So to use your example, load a 10mm to the velocity of a .40 S&W and it will run at substantially lower pressure than the .40. If all this were not so there would be no benefit to larger capacity cases.

[44man]

It is easy. Small cases with fast powder peak fast. Increase the case capacity and the peak pressure from a slower powder is out farther. You might see a peak an inch or so out but look at the trail of the pressure line. If it drops at a lower rate, there is still powder burning to add to push on the bullet.
Small cases with fast powders will peak quick and drop to almost zero to depend on gas expansion from depleted powder.
Some say all powder is burned in an inch or two, not true. Look at how the pressure line after peak diminishes. Some powder burning after peak maintains prolonged pressure.
At peak, not all is done and finished.
I read about the fireball at the muzzle on short barrels saying it was gas igniting oxygen in the air. Joke-right? Is is still powder burning. Powder makes it's own oxygen, enough to consume ALL of the powder. Why does a long barrel have less fireball?
Why does a short barrel have unburned powder? It is because pressure is needed to keep the burn and when the boolit exits, the pressure drops to zero. You can drop unburned powder from the case. Lengthen the barrel to maintain pressure and all the powder will burn. Yes, powder can burn the full length of a barrel and keep adding velocity.
If not true, we would only need one powder for any gun from .22 to .460. I am tired of those that say all powder is burned in an inch or two. That is the definition of a BOMB!

[paul h]

A bullet is accelerated by the area under the pressure/time curve. It is the area under the curve that determines velocity, not the peak pressure. The area under the curve is affected by the type of powder used, the ratio of case capacity to bore capacity and the peak pressure. When you use a smaller case, the ratio of the volume of the case compared to the bore is greater so the pressure decays faster, and you have less powder in the case which produces less high pressure gas to accellerate the bullet.

Don't confuse pressure with powder burn, while the powder may be completely burned up within the first inch or two of the barrel, the pressure does not drop to zero when the powder stops burning. You still have the high pressure gas which is pushing the bullet out of the barrel until the bullet leaves the barrel.

[sundog]

If the bullets/boolits are different when they leave the muzzle, even though they may have started out as the same bullet/boolit, they are no longer the same, ergo, a difference can be expected. Even a 'dumb', red neck, country boy in fly over country can figger that.

[44man]

Good points. I have done a lot of work with fast powders over the years, thinking about initial, fast pressure rise. Even my light .44 and .45 Colt loads of 7 gr of Unique showed a doubling of accuracy when I made the boolits much harder. Going to 10 gr showed even better and groups at 50 yards were much smaller then 25 yards.
Accuracy seemed to peak at 28 to 30 BHN. I kept changing the alloy and as the boolits got harder, groups got better.
Going back to hunting loads with 296 showed 18 to 22 BHN as enough with no special alloy other then WW's water dropped was needed. Air cooled gave me too many fliers.
Now if the boolit is too small in diameter, bump up will stop gas leakage but not once did it improve accuracy. Even the .475 and .500 thrives on WD WW metal.
Even when working with .58 muskets shooting Minie' balls, lapping the mold for fit at the muzzle would turn a rifle from missing a 4X8 paper at 50 yards into a 200 yard shooter. Skirt expansion was never even with loose boolits.
I do think different and can't tell you to slump a boolit to fit the gun.
Some boolits like a wad cutter are pre-slumped and not much better then a hunk of lead wire. Everyone shoots dead soft wad cutters cast with care but when the GG's vanish from slump, why lube them? Many think a coat of lube on the outside will last for the barrel length like a .22.
I have been called a crazy old coot but when I see a jacketed bullet shoot tiny groups, my idea was to try and make lead act the same. I have out shot rifles to 500 meters with revolvers, cast in both. It is why a rifle needs harder boolits as you increase pressures.
Why resist it with a revolver? The closer to the brass of peak pressure means an alloy to withstand it. It is why pistols like a 10MM or .40 need hard, instant pressure that is very high.

[Silver Jack Hammer]

Still the same velocity with the same boolit with such different chamber pressures? The only difference is the length of the brass? Can it be said that COL is more critical in short little stubby high pressure cartridges such as the 9mm and less critical in the longer .38 Special? It would seem if this were true then either folks like us or the ammunition manufacturers would tout the value of longer cases in revolvers for reaching higher velocities at lower pressure.

I recognize the reality of the consumer market is all about high pressure rounds like the 40 S&W and 9mm, while the .45 Colt does the business with heavy boolits and thin chamber walls. It's a new concept to me that a larger case can deliver the same performance at lower chamber pressure.

I don't own a 9mm to do my own testing at home right now, I've sold all the 9mm's I've ever owned, and I don't have the equipment to test psi or cup at home.

As I peruse the Speer manual 9mm SAAMI specs max cup is 35,700. 38 Special standard working pressure is 18,900 cup, plus P working pressure is 22,400 cup. The manual gives comparable velocities for 125 gr boolits in the 950 to 1000 fps range.

Very interesting. It seems there is room to develop a .454 Casual round on a frame size comparable to the new Ruger single action that would throw a 300 gr boolit or Mr. Skovill’s 270 gr Keith .45 boolit at higher velocity and less chamber pressure than a .45 Colt.

[44man]

Yes, true, larger cases will have lower pressure but remember, most can use a slower powder to reduce air space too. Would you shoot Bullseye from a 45-70?
The .45 Colt does NOT have thinner walls but shoots as fast with lower pressures.
The real difference is the speed of the powder needed with short brass. You just can't fit slow powder in the tiny cases.
Slow powder DOES burn past pressure peak. It does add to gas expansion. I fight tooth and nail over this but powder makers have proof.
Look at it this way. Take a .300 Weatherby with a max load of 4350 and also a max with 4831. There is more 4831 so if it all ignited in the same place as 4350, pressure peak would be HIGHER then 4350. Not so, pressure is lower for more velocity. A longer barrel is better with 4831. As you shorten the .300 barrel, faster powder is needed to reach the same velocity.
If all powder burned in the first inch why not use Bullseye in the .300?????????? Even gun rags say the wrong things because they only look at pressure peak, never beyond it. If they were right a .300 would only need a 2" barrel.
Sure a full charge of 4831 makes more gas but if done all at once, the gun would blow up.
Some experts do not understand "progressive" burn. 4350 can be the exact same powder as 4831 but size, shape and coatings are different to control burn rate.
If you told me 88 gr of 4831 burned in 2" in a .300, I would call you a stupid fool full of shrapnel.
Revolver and pistol shooters think the rules do not apply. They think a full charge of 296 burns the same as Unique. If so, why not use 24 gr of Unique in a .44? Why not 24 gr of Bullseye?
I have been bashed and smashed over this but all of you are smart and understand why we have different powders and burn rates. NONE burn in the same spot, all at once.

[35 Whelen]

Allan Jones in Shooting Times Feb 2013 talks about the same boolit “slugging up” differently at the same velocity when fired out of two different cartridges, the 40 Smith and Wesson and the 10mm. He states that the FBI’s Firearms Training Unit found the same thing the Speer laboratory found, that the same boolit gives different terminal numbers when fired at the same velocity out of the two different cartridges. Allan Jones states the Speer lab found that the two cartridges deliver the same velocity at two much different chamber pressures. The test involved the 180 gr .40 S&W at 980 fps. The 40 S&W needed 31,000 to 33,000 psi and the 10mm needed only 24,000 to 26,000 to reach the same velocity. What was discovered was the effect on expansion when bullets “slug up” under pressure. “The bearing surface lengthens at the expense of the nose length due to in-bore friction. The greater the pressure, the more nose shortening occurs. It’s very noticeable in boolits with pure lead cores…” Compared to the lowered pressure 10mm Lite, the 40 S&W pressures shortened the nose more and reduced expanded diameter, a critical parameter in the FBI’s calculations.

Didn't we ALL already know this? A 300 WM is capable of driving a 180 gr. bullet a given velocity with lower pressure than say a .308 Win. Right?

Hmm....Sounds like they could've saved a ton of taxpayers money with a call to Veral Smith. Wasn't he the one who came up with the formula regarding pressure vs. obturation?

35W

[Silver Jack Hammer]

It's been great, thanks for the posts.

BTW 44man the thinner walls I was referring to are on the cylinder of the Colt SAA, not the brass.

Thanks all.

[44man]

It's been great, thanks for the posts.

BTW 44man the thinner walls I was referring to are on the cylinder of the Colt SAA, not the brass.

Thanks all.
OK, very true.