Hmmm...
Never had a 16" AR have a problem (at least one w/upper from Colt or Bushy or J&T). Your post doesn't really give enough details -lotsa ways failures happen - failure to feed, failure to extract, failure to eject, etc. And is this a real Bushmaster or is it a parts gun with just a Bushy lower?
When friends or acquaintances have AR probs it's usu due to [in no particular order]:
1. inadequate cleaning before use
2. dirty chamber
3. inadeuqate lube
4. ammo probs (esp reloads or weak Brit ammo)
5. bad mags (non USGI, or bent lips, weak
spring, bad follower, backwards reinstall
of spring/follower)
6. non-chrome-lined barrel or non-
milspec "match" chamber
7. gas leak around the front sight/gas block
or at the carrier
CLEANING: ARs can shoot all day long w/o cleaning, but put 'em up for 2 mos w/o cleaning and problems can occur. Clean after range session and update lube periodically so you're always ready.
Take bolt apart and clean the hell out of it. Clean bolt carrier too. Bolt+carrier interface area should be wet with CLP. Use chamber brush to scrub out the chamber!!
LUBE: Inadequate lube of AR is another common problem. Is he using CLP (BreakFree) for lubricant? Also you may have jumped from the frying pan to the fire going from "too wet" with lube to too dry. I like to see a bit of shine on bolt carrier surface. Other gun oils, WD40, etc are not necessarily good for ARs. (Although I use Tetra also w/o problems).
MAGS: Aftermarket commercial mags are known to cause problems (esp steel USA mags). Stick w/USGI mags in good shape. If necessary install fresh spring and new green follower. (Note: springs in mags can be installed backwards, leading to problems. Try several different known-good mags from friends. Check TM for mag assembly.)
AMMO: Reloads often are problematic. Win Q3131 ammo has caused people problems. Russian varnished steel case ammo causes some folks probs (although I shoot it all day long).
Brit SS109 ammo is too weak to cycle many ARs.
NONMILSPEC CHAMBER/NOT CHROME LINED: Many ARs that have "match" chambers have trouble feeding military ammo. Rough chambers (dirty or lack of burnishing) in combination with tightness can cause failure to extract probs.
GAS LEAK: check for crud around frontsight/gas block where gas tube enters: could be a gas leak. Also, is carrier key loose from bolt carrier? Check that it's tight and that it's been "staked" around the screws.
First thing is to get into "baseline" situation and use quality ammo for testing. This means IMI, Federal Amer. Eagle, or Winchester Q3131A (*not* Q3131!). Use known good USGI magazines, and start with rifle that's been cleaned fully/properly. Follow the Army/marine tech manual debugging tree.
Problems in lower shouldn't really cause these problems, but just for heck of it swap a known good bolt + carrier into this rifle and see if problem persists. Swap your complete upper onto another lower (observing legal pre- vs. post-ban issues) to see if problems stop. If swapping lowers cures problems, check/replace recoil spring, and check fire control parts for proper function (see tech manual). Check if disconnector spring is installed improperly; check for proper positions of hammer & trigger springs (sometimes ARs will kinda function for a bit with these installed in screwy fashion).
If you address these issues you're sure to find the problem.
Bill Wiese
San Mateo, CA