View Full Version : pros and cons of different barrel lengths on an ar pistol?

07-15-2010, 12:57 PM
i've seen some people say that for 100% reliability a ~10" barrel is best in order to get enough dwell time? can anyone comment in detail on this?

i would rather go with a 7.5" but not at the expense of proper function. i'm going to be using .223.

if 7.5" has function issues, can that be solved by using an adjustable gas block?

07-15-2010, 1:57 PM
I'm in the same boat. My next AR build will probably be a pistol, and I was set on the 7.5" until I read about reliability issues as well.

07-15-2010, 2:20 PM
I have shot 7.5" before......fuctioned just fine. If you're building you're own you might wanna check into proper gas port sizing / tuning beforehand.

07-15-2010, 2:45 PM
I can't explain to you the detailed engineering behind it, but I recently picked up a Spike's Tactical 7.5" pistol, and after putting about ~200 rounds through it, I have not experienced any cycling issues that would point to a mismatched gas system/buffer. The only problem I had was that the bolt was locking back even with rounds left in the magazine, but it was quickly solved by taking off the Magpul BAD lever. (With just the little added mass, I guess the bolt catch was getting jarred upward.)

There's scant documentation on Spike's website, but from what I understand, the barrel has a slightly widened diameter gas port, and the buffer is heavier, which allows it to work without muzzle devices like the STAB (http://www.spikestactical.com/z/index.php?main_page=product_info&cPath=63_67&products_id=303), the Noveske KX3 (http://noveskerifleworks.com/cgi-bin/imcart/display.cgi?item_id=kx3556p&cat=11&page=1&search=&since=&status=) or the PWS CQB Comp (http://primaryweapons.com/store/pc/viewPrd.asp?idproduct=90&idcategory=6). The main issue with these shorter barrels is the reduced back pressure, and supposedly these muzzle devices correct that.

Another problem arises out of using the modified buffer tubes like the GunSmoke Enterprises one. They certainly look cooler, but it adds to a lot of the headaches. The Spike's lower on my pistol came with this: http://www.spikestactical.com/z/index.php?main_page=product_info&cPath=104_97&products_id=69 which is supposed to be a standard carbine length tube with a H2 buffer.

My bigger concern with these pistols was that the muzzle velocities are considerably reduced - using FMJ out of a 7.5" barrel, the best that could be hoped for was around 2,200fps, and that's too slow for full fragmentation. Even the small bump in length to 10" helps considerably. Another solution, of course, is using OTM or other hollow point ammunition.

Check out this chart from AR15barrels.com showing the different gas system lengths and muzzle velocities: http://www.ar15barrels.com/prod/gas-systems.shtml

My AR pistol is loud as hell and is quite a fire-breather, but it's the most fun I've had with a firearm. :D

07-15-2010, 7:15 PM
Longer barrels give you more bullet velocity. Randall (AR15barrels) had an excellent chart which showed exit velocity wrt barrel length...it is in the link above...

07-15-2010, 8:24 PM
I'm running a 7.5 barrel on mine. I'm using a RRA pistol buffer tube. It's basically a carbine length buffer. The guys running real short tubes are having cycling issues. Right now i'm using a regular A2 flash hider. It's not hiding much flash. Nice fireball in the daytime and super loud. :83:

07-15-2010, 11:37 PM
ive got a addax/pws diablo 7.5" upper with a gun smoke 4" pistol buffer and ive got no problems cycling at all. if your building the upper yourself then i could see a problem with improper gas port size but if its built by a reputable company they should have all that taken care of.