I'm hoping someone here can help me.  I just got back from the range.  My bushmaster fired three rounds semi-auto and subsequently became a straight-pull bolt-action.  The is a 16" Bushy M4 with about 300 rounds through it.  The bolt would not fully recipricate.  It would start reward movement and then stop w/ the spent case extracted only half way out of the chamber.  Does anyone know what causes this.  I don't think it is the mags because it did this with several mags, all of which worked flawlessly in my Colt MT6601C.  BTW, the bolt carrier stayed in its semi-open position after the magazine was removed, so I don't think it was hanging up on the mags.

Come on guys, some has gotta know something.

Break the Problem down--

There is:

A: Not enough push from the Gas to move the bolt/carrier back, or

B: Something resisting the push from the gas.

Clean the gas tube, make sure the buffer spring doesn't bind, make sure the Bolt/Carrier moves freely, and change ammo to try to see if that is the problem.  A thorough cleaning may solve the problem.  Check for any pieces of metal from the manufacturing process to see if that is binding anything.

You could swap uppers to see if it is the buffer spring, swap bolt/carriers (check headspace before firing, though).  Try to isolate and replace each component to try to solve the problem.

AFARR

AFAAR, I did everything you suggested at the range.  The one little detail I over looked was the gas key.  So much for staking those screws down.  I just notice the gas key is flopping around a bit, which obviously means gas was escaping where the key meets the carrier.  Hence, not enough gas/force was being applied to the carrier to enable it to cycle properly.  

theirs a victory, open and shut in 3 hrs.

slt223 you may want to replace the screws. I have been told that the screws are weakend when loose and then shot. They then can shear off easier.

I knew a guy who put together an AR and didn't stake the screws on the gas key. About 100 rounds into shooting the gun did exactly what you experienced, only in his case the screws not only loosened but they did shear off. Luckily there was just enough of the screws left sticking up from the carrier to grab and twist out with needle nose pliers.