What kind of ammo are you using?  

The failure to feed and failure to lock back problems, combined with your description of [b]"the bolts really seem to go back very slowly and with little force"[/b] make it sound like you are using either a lightly loaded round, or the gas port is too small.

Sounds like they're short stroking. The 11.5" barrels are part of the problem. They're finicky and the gas ports must be just right or it won't fully cycle the bolt. These rifles were originally designed for a 20" barrel. With 11.5" barrels there isn't much bore available past the gas port and reliability suffers. In short barrels, the gas pressure is a short spike vs. a longer, more consistent pressure spike with 20" barrels. This is one reason I'm not a big fan of short barreled rifles, along with the velocity/fragmentation range issue. Yeah, 11.5" barrels have the "cool factor" going for them, but IMHO, I wouldn't have less than a 14.5" barrel. Sorry if this doesn't help you much since your dept. already bought them. I hope Armalite makes it right for you.

People have been having problems with AR15 type rifles with barrels shorter than the Military M4 (14" length) although the shorter barrels are commonly available.  The most common problems relate to reliability problems.  One expert on the subject had posted a very good essay on solutions to the problems and I have copied it to this page.  

The 11.5” barrel was used by the US Army on the XM-177. The M4 carbine later superseded this rifle. The 11.5” AR is a finicky animal. Sure some particular rifles will work in the semi-auto mode, but they are not reliable enough in full auto. This lack of reliability and its reduction in muzzle velocity did the XM-177 in.

The inherent problem with this set up that the gas port is to close to the muzzle. The AR-15 runs on the direct gas impingement system. Whereas a long stroke gas piston will work with the port close to the muzzle, the direct gas impingement system will not achieve reliability that is acceptable in military conditions, with the port close to the muzzle. The direct gas impingement system runs on the pressure behind the bullet after the bullet passed the gas port and before it exits. The Marines (US Army did not fund it for length of the program) funded the XM-4 program in order to get a more reliable shorty. It was found that the AR-15 needs at least four inches between the gas port and muzzle. This was determined after extensive testing. You will note that the 14.5” barreled M4 and the 20” M16 have the same distance form the port to muzzle.

Many AR-15 barrel makers and gunsmiths will attempt to correct this problem by drilling the gas port larger. This will result in increased bolt velocity. The bolt velocity achieved in current AR-15 type rifles is already faster then Stoner had intended. With increased bolt velocity in the AR-15 the extractor can skip the rim or rip it off. If the case is indeed extracted form the chamber intact, then problems can show elsewhere. The bolt will rebound heavily off the rear of the buffer tube and can come back over the mag before another round is stripped out of the mag. Once the bolt group strikes the barrel extension it will rebound farther back due to increased velocity and momentum. This can lead to the hammer falling while the bolt is unlocked. Normally this will simply show up as a miss fire in case of the later rebounding problem.

In short the problem is not port pressure that can be solved by port size. It is a time problem. Some reliability can be gained by using a heavier buffer. The M4 has been improved with the addition of the “H” marked buffer. With the larger port that is common on an 11.5” barrel the bolt group will be pushed hard for a short time. The heavier buffer can slow the bolt group down. A proper heavy buffer will also help in holding down bolt group rebound by absorbing some of the momentum. A better solution to using a heavier buffer is to use a standard length buffer tube/stock assembly. This will also reduce bolt velocity and corresponding bounce back.

I recommend simply replacing the barrel with one that locates the gas port farther away from the muzzle. Buy one that is Mil-Spec as in FN, Colt’s or Bushmaster. These barrels have chambers that are cut appropriately for use in an AR-15. The chamber dimensions in any gas gun are very important for reliable function. It would seem that your gas port is too large for a heavy buffer to fix. You could try a full size butt stock as a cheaper test.

The above info is from [url=http://www.savvysurvivor.com/gun%20pages/constructing_and_gunsmithing_the.htm]here[/url]. Couldn't get it all in due to the 3500 character limit. HTH

A little sensitive are we? Chill out, I didn't see any harm in his question.

Merost,

Sorry but all the armalites vs. bushmasters I have personally seen, the armalite beats them out in fit and finish. You would have the same problems with an 11.5 inch bushie as well. And I don't believe that you are getting sufficient velocity to penetrate decent armor as well. Just my opinion. Contact ArmaLite, they will help you.

dave