I am looking into building the Cav Arms 45 AR with the Oyl arms upper. I'm wondering what the difference in velocity would be between the 11.5" barrel with 5" flash hider, or the 16" barrel with the 45 round. I can't imagine it would be all that different, but I don't know all that much about it. Would I loose any signifigant amount of velocity with the 11.5" barrel? I kind of like the looks of it more is the only reason it's even an issue.

Anyone?

I’m no expert, but I have read that the 11 in barrel will actually produce greater bullet velocity than the 16.  The bullet has already achieved maximum velocity from the available powder by 11 in, and the extra 5 inches of barrel actually slows it down because it adds resistance.

Quoted: I’m no expert, but I have read that the 11 in barrel will actually produce greater bullet velocity than the 16.  The bullet has already achieved maximum velocity from the available powder by 11 in, and the extra 5 inches of barrel actually slows it down because it adds resistance.

Can anyone confirm this?  

No hard data unfortunately, but that is what i've always heard. The same goes for 9mm too, a 9mm +P+ will have pretty much equal velocity coming from a 6 inch barrel as from a 16 inch barrel. So somewhere in between there is a point of diminishing returns. I'd go with the shorter barrel.

The bullet still comes out of my 16" .45.  It hits the steel very hard.  I have never measured the velocity, nor lost sleep over it.

I had a pair of .45s a long time ago, one a 11.5" with extra-length flash-hider, and a 16" The velocity of the 16" was not less than that of the 11.5", but it wasn't enough more to bother over. Now, that was with factory ammo. Reloading, and using slower-burnign powders, you could probably get a velocity increase, but at what cost? You'd simply be pounding the carbine for a few more fps.

9mm is different. It starts at enough-higher pressure that you get a sustained push through the full 16" of barrel. A 16" is markedly faster than a 11.5", all other things being equal.

However, not all things are equal. Some barrels are faster than average, and some slower. If you happen to have a "fast" 11.5" and a "slow" 16" you may not have any difference in velocities.

In the end, do what makes you feel good.

45 ACP and 9mm are pistol rounds, and pistol rounds are typically loaded with a very fast burning powder that's designed to be fully burned up in pistol length barrels, thus no huge flash when shooting out of a pistol.  Shooting them out of a barrel any longer than 6" is not going to get you any more velocity unless you roll your own and substitute more of a slower burning powder for the fast burning pistol powder.  Then you run into case capacity problems.  Which is why large case rifle cartridges were developed, to contain more of a much slower burning gunpowder to make use of long barrels.

Some pistol rounds, .357 Magnum and 44 Magnum come commercially loaded in a variant meant for rifles.

I'd love to have one of the Oly AR's in .45 ACP, but I'd want to SBR it.  10"-12" is the way to go and no flash supressor is needed.  No compensator will probably be needed unless you plan on putting a happy switch on it.

Post pics when it's done!

I've had the same question, and I have a 16" .45ACP setup.
What I have in mind currently is to have the bbl cut back so that when the FH is perm attached the OAL is 16".

I zero'd the rifle at 50yds, and at 100yds, I'm hitting about a foot low with Win White Box.

I don't have a chrono, or I'd have checked the velo already.

if it isn't going to be a SBR there is really no point

Quoted: if it isn't going to be a SBR there is really no point

Ok, I ordered the upper today. I went with the 11.5, and flash hider. I am going to make it a short barrel eventually though. I'm waiting until my fiance and I settle in some place. I already have a registered MAC, and a 22 silencer, and they are a hastle to move around. So no more class III until I own a house. I will definately post pics when it's finnished. I can't wait to get this upper. Now I need to save up some cash for the lower, and the other stuff.... heavy buffer, and mag catch.

Keep in mind that two identical guns with the same barrel lengths can still shoot to different velocities.  I have chrono data for a few different barrel lengths.  It's not a factory load, but it's a handload that's close enough.

Springfield 1911 - 5" barrel: 811 fps
MAC10/45 - 6" barrel: 821 fps
Marlin Camp 45 - 16" barrel: 916 fps

I did get soem gains in the longer barrel by going to a slower powder.

The 1911 got 791with the slow powder load and the Marlin did 1022

Standard pressure 9mm is equivalent to .357 MAG coming out of a 16" barrel. Unless you're shooting FMJ's, stick with standard pressure JHP's because most of them are designed to expand at the lower velocities.

Quoted: Standard pressure 9mm is equivalent to .357 MAG coming out of a 16" barrel. Unless you're shooting FMJ's, stick with standard pressure JHP's because most of them are designed to expand at the lower velocities.

I would love to see some chrono date to back this up.

Quoted: No hard data unfortunately, but that is what i've always heard. The same goes for 9mm too, a 9mm +P+ will have pretty much equal velocity coming from a 6 inch barrel as from a 16 inch barrel. So somewhere in between there is a point of diminishing returns. I'd go with the shorter barrel.

Quoted: 45 ACP and 9mm are pistol rounds, and pistol rounds are typically loaded with a very fast burning powder that's designed to be fully burned up in pistol length barrels, thus no huge flash when shooting out of a pistol.  Shooting them out of a barrel any longer than 6" is not going to get you any more velocity unless you roll your own and substitute more of a slower burning powder for the fast burning pistol powder.  Then you run into case capacity problems.

For 9mms, that is incorrect. There used to be a website site now down) that had test results from shooting various brands of commerical ammo through pistol and pistol caliber carbines. In some cases, the chrony showed that 9mm rounds gained upto 30% velocity from a 16" bbl. There is also a thread in the Rimfire and Pistol Caliber forum (which is where this post should be) in which a board member did their own testing and found similar results.

I don't know how bbl length affects the .45 ACP so I won't venture to guess. Unless someone has hard data, we're all just "guessing" which unfortunately does not answer the OP's question.

I had a rep from Oly tell me it is OK to shoot .45 Super in their .45 guns but it would void the warranty. I know the .45 Super is hard to come by but it would solve any loss of velocity issues.

Shooting .45 ACP +P ammo should gain back any lost velocity from standard rounds also.

Quoted: I had a rep from Oly tell me it is OK to shoot .45 Super in their .45 guns but it would void the warranty. I know the .45 Super is hard to come by but it would solve any loss of velocity issues. Shooting .45 ACP +P ammo should gain back any lost velocity from standard rounds also.

That's assuming that velocity is lost when shooting it from a 16" bbl.

Again, it's one thing to hypothesize that .45 ACP fired through a 16" bbl loses velocity but I won't buy into that until I see hard data (chrony results) first. The test results for the 9mm carbine was definitely an eye opener.