Gun is a DPMS AR 15, SSA trigger, free float tube, AGB, Luth AR Carbine stock, Sightron SIII. Load of XBR, Rem 7-1/2 primers, 77hpbt at 1.856 ogive, or 2.250, (just under mag length for me) 23.7-24.5 on the powder. Shot off rear bag and Harris bi pod. Barrel has 125 rounds through it. It started...