I had this same problem with a brand new DPMS rifle that I purchased about 2 years ago. After a phone call to DPMS (who, in turn, stated that I need to use American ammo and not the cheap russion stuff even though I only use Winchester which is made in Israel) . In any case they wouldn't help me, even after numerous calls so I sold that rifle and bought a bushy and have had ZERO malfunctions.
A friend of mine had a similar problem with a used rifle that he just recently bought, no it wasn't mine but is was a DPMS, and he in turn brought it to a gunsmith who said the chamber was out of specs and when the case expanded it basically wedged itself in the chamber so tight that the extractor couldn't do its job and just basically ripped the cases. He eventually replaced the barrel and never had another issue.
If it is new contact the FFL it was purchased from or contact the manufacturer.
Hope this helps