First time at range with new rifle, first time firing. After 25 rounds of PMC 55 grain FMJ ammo, the bolt jammed part way open and was stuck hard. Removed magazine and knocked out both take down pins. Removed upper and the recoil buffer retaining pin was down in the action and the retaking pin spring had been cut in two. Very puzzling. Fortunately I had a spare spring. Got the parts out of the action, reinserted the spring and retaining pin. Not sure how the pin is retained...looked like a flat on the buffer is all that holds it, and a slot in the bottom of the bolt appears to ride over it. No staking, no retainer. Reassembled rifle, test fired and had same failure after third round. Why is this happening?