Barrel Length question for AR-15 Pistol caliber uppers

[Greywolf]

I have the SBR forms approved, and am now ready to get the 9mm AR upp and .45 AR Upper cut down for eventual suppressors.

9mm - what barrel length? I was thinking of 8"-10" - have read a lot, but no one seems to have THE answer - will be shooting 147 grain subsonic almost exclusively through a suppressor.

45 - Looks like 7"-9" again may be the best bet. Again, will use subsonic (duh) and mostly through a suppressor.



Any ideas, guys? I want to optimize my accuracy and fps, but given my subsonic parameters and suppressor use I want the barrel as short as reasonably possible to get the ballistics performance I want.

[fanoblack]

I have read that some Mfg of suppressors will not honor the warranty if it is fired on a bbl shorter than 11.5 or 10.5 I can't remember for certain.

This is relevant to the 5.56 uppers, I know realize after rereading your post that you didn't discuss anything but 9mm and .45. What are you looking for ballistics wise, most full sized pistols only have a 4-5 inch barrel, so are you wanting better than that or equal?

What suppressor do you have and how long is it? (if you don't mind me asking)

fanoblack

[Greywolf]

Basically, I want to get the most burn out of the powder before the extra barrel length starts to drag the bullet down again. Most of my research is spotty - seems Colt has the length at 10.5" for their 9mm Carbines. But if I can shave off another inch or two and still maintain the velocity/powder burn I'm looking to get, that would be ideal. The shorter the barrel, the better :D

[fanoblack]

This (http://isntapundit.com/?date=20030925) is just one site I found, but it says that the 9mm round will start to slow down due to barrel friction after 10-12 inches.(varies on 115-148gr.)

So I would say something in the order of 10 inches would do fine.

fanoblack

[shaggy]

Greywolf - I can't help you with exact numbers for velocity loss at those lengths, but I think the 10.5" 9mm is optimal. That said, I don't think you lose much velocity going back to about 7.5". I've had both Colt 635 and 633 SMG uppers and I prefer the shorter 633 (the 635 was 10.5", the 633/DOE gun had the 7.5" barrel w/M231 handguards & flip-up front sight). If you plan on adding a can, I'd definitely go with the 7.5" - the 3" barrel length you save on the front will help keep the overall length with the can down. Remember, most 9mm cans are about 6"; add that on top of a 10.5" barrel and you're past where you'd be with a 16" carbine.

[Greywolf]

I guess I'm just wondering if when I am shooting it without the can with a 7.5" barrel, am I hurting myself in a big way vs. getting a 9" barrel? Though again, I think for the most part I'll be sticking with the suppressor most of the time.

[shaggy]

I really don't think you'll lose that much velocity by cutting back even 3" from a 10.5 to a 7.5. Just a rough guess but maybe 100-200fps total? The importnt thing is that a 9mm isn't as reliant on velocity as a 5.56 to do its damage. I've had short 5.56 uppers but won't go below a 14.5" anymore because of the velocity loss. With 9mm, however, even a 7.5" barrel is going to do as much, if not more, than a 9mm pistol. Just consider a Beretta M9 has what - a 5" barrel? An M11 has a 5-6" barrel. How long a barrel does an MP5k have; 7" or 8"? Even the 9mm Colt 633 guns have a 7.5" barrel and from what I understand those were built for nuclear security personnel via the Dept. of Energy.