I have a 6921 upper w/ a DPMS carrier and as of my last trip to the range it turned into a single shot.   I looked at the top of my bolt carrier and noticed that one of the allen-headed screws had broken off.    Prior to this,  I have had zero problems  but would it be smart to replace the carrier or just replace the allen screw.   Also,  if I did replace the bolt with a new colt one,   what kind of headspacing issues would I have do deal with??    THANKS IN ADVANCE FOR ANY INFO

Replace both allen head bolts with new ones (refer to USMC TM 05538C-23 sec 3-11, pg 3-19/3-21) and make sure they are properly torqued or you will continue having problems.... As for replacing the bolt, if you do, check the headspace with a proper gauge to be on the safe side. Headspacing is a function of fit between the bolt and the barrel extension and when you replace one or the other, headspacing should be checked.

Good Luck....