Tommy Gun Conundrum

['88Scrat]

So I watched an episode of American Guns last night, where they made a custom M1928 for a customer who was a competitive shooter. They cut down the barrel from "18.5 to "16.5, did a trigger job, adjustable stock, and even a titanium reciever. It was cool.

But one thing I don't understand is how they got a higher fps out of the "16.5 barrel (853 fps) then the "18.5 barrel (802 fps). Shouldn't the longer barrel facilitate a higher fps rating?

 

[Aguila Blanca]

A Tommy gun fires .45 Auto, which is a handgun round designed to be fires out of the M1911 pistol, which has a 5" barrel. Much depends on the powder being used. Velocity increases in a longer barrel as long as powder continues to burn and build pressure while the bullet is in the barrel. Once the combustion ends or slows down to the point that the increasing volume resulting from bullet movement offsets pressure increase, then additional barrel length only adds more friction, and more friction reduces velocity.

The optimum barrel length is going to vary somewhat depending on the exact load being fired.

 

[Closing The Gap]

Although I only watched the show once or twice I feel its a shame that Discovery let the pressure of antis get to them. They cancelled that show just after the shooting back in December.

As for the higher fps I'm no expert but I thought I'd seen something posted before similar to this. The barrel length beyond optimum can slow the bullet from what I understand. Maybe since its a Tommy gun which uses a pistol caliber designed for shorter barrels is the reason why it was faster out of a shorter one? I'd like to know for sure myself too.

Edit: oops had that sitting there and posted before I realized someone else had chimed in. Sorry.

 

[jtmckinney]

45ACP cartridge just runs out of power in that length barrel, probably even before 16.5 inches.

I handeled one of these at my local gun store yesterday. Heavy and did not at all fit me.

I have a Hi-Point Carbine in 45 ACP with I think around a 16 inch barrel I bought to hunt havelinas with. I am able to get quite a bit better velocity but I only use my reloads I wouldn't shoot in a pistol.

Hope this helps!
James

 

[44 AMP]

Another thing besides length, to consider...

There is more than just barrel length to consider. There is the barrel itself, and the amount of difference in velocity.

Different barrels, even ones identical in length, can produce different velocities. Usually the longer barrel delivers a higher velocity, but this is not always the case.

The fit of the barrel (bore) and the bullet make a difference. A "tight" barrel may deliver lower velocity than a "loose" one. I have seen a 4" revolver actually deliver slightly higher velocity numbers than an 8" one. Not usual, but not impossible, it depends on the specific gun and ammo used.

Also, there is the amount of the difference. 50fps or so is within what I consider the normal range of diffference between different guns. I have seen 3 different guns, firing the same ammo, and with the same barrel lengths deliver as much as a 100fps spread in velocity. This is uncommon, but it does happen, depending on the specific guns (not the models, the individual guns) tested. Usually the velocity spread is 50fps, or less, but not always.

The idea of the round "running out of steam" in the longer barrel, because it is desgned for a pistol seems logical, but I don't think that is what actually happens. Yes, one can reach a point where all the powder is consumed, and there is no increase in the pressure pushing the bullet. And, in theory, after this point, barrel friction will affect the velocity more.

But nothing I have seen indicates that the bullet will slow down, rather what happens is the rate of velocity increase per inch of barrel goes down. And, this exact number will be different for each individual barrel.

Chronographs are neat things, and fun to play with, now that they are affordable to most of us. And (good ones) seem pretty accurate. One thing they do show us, if you use one enough, with enough guns, is that each gun may be a law unto itself.

Generally, like guns shoot in a similar group, but there are individuals that may be faster, or slower, with the same ammo. Sometimes the difference is significant. Most of the time, it isn't.

Published data is only a rule for the gun(s) and loads used. Those individual guns. For all other guns, its a guideline, (Arrr!) and while usually close, may not be what your actual gun and load are delivering in your hands.

A shorter barrel being faster than a longer one isn't typical, but it happens, sometimes.

 

[Aguila Blanca]

Try this: http://www.ballisticsbytheinch.com/45auto.html

Note that the maximum velocity for each different cartridge is not always achieved with the same barrel length. For example, the first one listed (Cor-Bon 165-gr +P) keeps increasing right up to the 18" end of test. However, skip over to the next-to-last entry (Federal 230-gr Hydra-Shok) and it gets maximum velocity out of the 11" barrel, and from there the velocity decreases as barrel length increases. But this experiment used all premium self-defense rounds. I suspect that a standard 230-gr FMJ loaded more or less to military specs (approximately 830 fps out of a 5" barrel) would also peak at less than a 12" barrel, and would see velocity fall off considerably thereafter.

 

[44 AMP]

interesting link, thanks.

Going down the page a bit further, and looking at the "real world" results, using the same Fed 230gr load, you can see slightly different velocities out of three different 16" barrel guns, and where a 3.625" barrel gun turned in a slightly higher velocity than a particular 5" barrel gun.

The actual velocity difference is insignificant, other than from an intellectual viewpoint. 50-60fps at pistol speeds makes no discernable difference in impact performance. It just makes a difference when comparing absolute numbers of fps, nothing more.

Looking at these numbers actually shows one real thing, and that's my point, while there are significant trends, it is the actual individual gun & ammo combination that determines the velocity you get from a given load. And what you get from yours can be quite different numbers than what a factory test, or the guy next to you on the range gets. It is usually not a significant difference in down range performance, but the odds of your numbers being identical to someone else's are slim. And since down range performance is what matters, differences of a few fps are only of academic interest.