Mark,
A couple of thoughts regarding your carbine problem.
Is the bolt trying to feed the next round into the chamber when the malfunction occurs?
If the answer is no, you may not be getting enough counter-recoiling force. The likely causes are a stuck / frozen gas piston or fouling in the gas port. The piston nut should be removed (using the special wrench) and the piston cleaned of carbon with something like Krol oil. The gas cylinder should have a slight chamfer (see TM-9-1276). The gas port should be .070 in diameter. Ream it clean with a .070 diameter drill bit by hand.
If the bolt is trying to pick up a new round you are back to the bolt / chamber area. First I would replace all the bolt components, extractor, ejector and springs. Finally I would inspect and polish the chamber.
I have used the above methods on two different guns in the last tweleve months and now they both run problem free.
Please let us know what you find.
Jeff