Re: DPMS problem?
Could be that the gas block wasn't properly aligned by the factory.
Could be that the buffer spring needs changing.
Could be that the ammo is the culprit.
Bushmaster & DPMS (and Remington, Cobb, Marlin, AAC, etc...) are all owned by Cerberus. Now, what that means as far as manufacture is concerned(?) I don't know. I know that the R15 from Remington is the Bushmaster Predator without the phosphate coating & with a regular crap trigger.
I had some PMC ammo that was causing the same issue - shoot several types of ammo and then move to a different probable cause.