Optimum Barrel Length for .45acp

A

amprecon

New member
I've been looking at carbines in .45acp and got to wondering at what point does barrel length start to inhibit the velocity of the .45acp cartridge. I think the Tommy gun barrel length was 10.5" (?), was that the optimal length before barrel friction started to slow it down? The H&K UMP .45acp submachine gun barrel length is just under 8".
Since minimum barrel length for rifes/carbines is 16" without going through all that BATF crap, I'm not going to buy a .45acp carbine if 16" of barrel length starts to slow down the .45acp before it even leaves the barrel.
And no, I'm not interested in other calibers.
 
Thanks wnycollector, insightful, yet curious, especially as many loads peak at a shorter barrel length, decrease velocity between several longer barrel lengths then speed up again as the barrel lengths get even longer.....peculiar.
 
curious, especially as many loads peak at a shorter barrel length, decrease velocity between several longer barrel lengths then speed up again as the barrel lengths get even longer.....peculiar.
Click to expand...

I would guess it has to do with the fact they used 10 different firearms to collect their data. When you average and then extrapolate the data you see the little glitches more. I find the "real world weapon" tables give more reliable numbers.
 
Also interesting is how different 'real world' weapons got slightly different muzzle velocities with the same length barrels. This goes to show that other factors contribute to velocity. A barrel with a lot of freebore before the rifling starts will shoot a different velocity than a barrel that has almost no freebore. A tight .451 bore will shoot a different velocity than a loose .454 bore.
Anyway, the difference between optimum and 16 inch barrels does not seem to be very great with any of the ammo tested. For some of the ammo, 16 inches was pretty much the optimum barrel.
 
P-P, yeah, that was taken with me cutting with a dull blade. We discovered that we needed to change the blades a lot more often than anyone expected - the steel used in the barrels is pretty tough.

By comparison, the steel used in the Uberti clone we recently tested was more mild and there was a lot less of it - meaning that the chops only took a few moments.

Cheers -

Jim
 
There is no practical barrel length where friction actually slows down the bullet. This might happen when you get to barrels several feet in length, no one has done tests on barrels 6,7, 8 feet or more, that I know of.

What happens with the small capacity cases is that the rate of velocity increase per inch of barrel decreases, once you get past the point where all the powder is burned. You still have the bullet increasing in velocity, until it exits the muzzle, all that changes is the increase in fps/inch. They don't slow down (lose velocity) due to friction.

16" and 18" barrels are found on pistol caliber carbines, and as far as I know, at these lengths, velocity is still on the increase when the bullet clears the muzzle.

Looking at the charts in the posted link, I see several instances where velocity remained virtually the same, or even decreased slightly, with certain loads and barrel lengths. But all were within 10fps, which is well within the shot to shot variation found in regular ammo. Do not get hung up of tiny velocity changes, they are insignificant.
 
44 AMP said:
There is no practical barrel length where friction actually slows down the bullet
Click to expand...

I was playing around with a chronograph and a Marlin 1894 in .357 Magnum when I decided to feed some .38 Special wadcutter target loads. These give me 650 fps from my 6" revolver. When I put them through my 16" Marlin, the chronograph showed that I was getting 606 fps from the carbine with that load.
 
PawPaw, that is truly unusual performance. I have never seen that happen with a carbine barrel, but obviously it did. I have seen shorter barrel pistols shoot faster than longer ones. Its not common, but it can happen.

In those cases, it's not the barrel length that is the overriding factor, but the fit of the bullet in the bore. Some guns are just "faster" than others. Sometimes, by just a little, sometimes by a fair amount. I have seen three different 6" .357s, shooting the same ammo, turn in a 100fps spread between the slowest gun, and the fastest.

Lots of odd things can, and do happen, so I would have to go with, in this case, "its the exception that proves the rule".
 
I discussed the low velocity from a carbine issue with a friend today, and he informed me that he has seen it happen, more than once, but only with lead bullets, not with jacketed slugs.

This seems to fly in the face of logic, but apparently it does happen. He puts it down to increased drag on the bullet from the longer barrel. Again, this seems contrary to accepted wisdom, as we have always heard about how jacketed bullets have greater friction in the barrel than lead bullets, copper alloy being so much harder than lead, and all.

But it seems that reality is somewhat different. My experience with jacketed bullets never made me realize that lead bullets (which I have shot a lot of, but not out of carbines) might not follow the same rules.

My friend also pointed out that it is possible with an ultra light load to have the bullet exit a pistol length barrel, but get stuck in a carbine length barrel!

So, it appears that, depening on the load, there is a length where the bullet would be slowed down by the barrel, although I have no idea how long it would have to be for a jacketed .45ACP bullet.
 
I remember a test report on a .22 long rifle barrel being shortened in increments and the velocity increasing as the barrel was shortened until it maxed out at about 14 inches and then dropped as the barrel was farther shortened. This was published in some gun magazine back in the '70s.

I chronographed .22 standard velocity ammo from my 7 inch barrel pistol and my 24 inch barrel rifle and the rifle only added about 69 fps to the velocity obtained with the pistol.
It looks like all that extra barrel on the rifle mostly just guided the bullet to the target.
 
I have chrono'd a lot of different 22 loads from 1-1/8" up to 18" of a 10/22 and the highest velocities obtained were from my buddys 10" MKII pistol. The same loads were slower from RUger 10/22, and a model 60.

In my friends Storm in 45acp we got consistently 150fps faster velocities in the 16" Storm over the 5" guns.
 
I wonder if 12 inch barrel extended to 16 inches with a permanently mounted bloop tube counts as a 16 inch barrel to the feds.
 

I believe it does if permanently attached.

I know that an integral supressor counts as inches, but a removable suppressor needs a 16" barrel on a rifle... NFA odities being different.

State law may vary.




-tINY

 
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