I just "finished" my first build. DPMS LR .308. Unfortunately, when I went to shoot it,I had a serious issue. 1st. I shot several rounds one shot at a time.No problem. 2nd. I began to rapid fire. After about 6 shots the rifle jammed with one round stuck half way into the chamber being held solid by the bolt carrier group. Wound up taking rifle apart and found that the Buffer Retainer and Buffer Retainer Spring had jumped out of their original position. The Retainer was jammed between barrel and Bolt Carrier Group. Retainer was bent and scuffed.The Retainer Spring was bent and twisted and was laying on top of the trigger assembly. I have ordered new Buffer and Spring. My question is WHAT WENT WRONG and HOW DO I FIX IT. I would appreciate any help/advice. Thanks