I was shooting the AR and having FTF problems.  I checked the gas tube and key and they looked OK.  I replaced the gas rings and changed the carrier.  I worked with new and old mags and the rifle would not always function correctly.  Today I noticed the bolt would be caught by the bolt stop now and again.

I noticed the bolt stop was not under good spring tension and removed it from the lower.  The detent was locked in the hole by some minor burring.  I removed the detent with a drill by hand to break the burr.  

I have reassembled it with new parts and I am going to try the rifle soon.

My question, has anyone ever had a problem like this and does anyone think this could be the crux of the matter?

On the bottom of the carrier, there is a slot that should allow it to cleanly pass over the top of the catch.  On a few rigs, including using the single loading sleds, the bolt hold-open can bind up on the carrier slot edge.  The simple solution is to just roll file the up most edge (when pressed up) to allow the carrier to smoothly pass over with out catching on the ejection port side of the carrier channel edge.  

As for the bur in the detent channel, I don’t believe that it should have caused you a problem in regards to the carrier hanging up on the catch.  The problem should had been that the catch would not rise far enough to catch the bolt once the mag went empty and the mag follow raised the catch. The pressure from the catch spring keeps the bolt catch down, with the mag follower overcoming the spring pressure and raising the catch to retain the bolt back.

What you may want to look at is the catch tab that is raised by the mag follower.  The tab may be a bit long and is catching the spring windings in the mag, and as the spring catches the catch tab, the carrier rides off the top round and the mag spring winding pressure is raising the catch up.

Hope this helps.

mwarshe,

Yes, I've seen that problem. Without proper spring tension and under recoil the bolt stop will jump up and can catch the bolt. Dano mentions a good a check, press in on the bolt catch and retract the bolt fully, you should not feel any binding, if you do, remedy is as he described.

Next time try to not change more than one part at a time, doing so makes troubleshooting the rifle difficult.

To clarify, are you using FTF as shorthand for "failure to feed" or for "failure to function"?

I wish to thank all for their response.  I talked with Bushmaster today and even though the person I spoke to never experienced this he did concur that it was possible.

The "FTF" would be failure to feed.

I will be trying it tomorrow..............

UPDATE:

The bolt catch detent hangup was the cause of the FTF.  The rifle cycles fine with no interruptions.  Thanks again for the input!