Shot twenty rounds yesterday from a Remington 1903-A3. Ten rounds were loaded with NOE 314299,sized to .311, Felix lube, gas checked atop 28 grains of IMR 4227. The other ten were Hornady 168 grain HPBT with 46 grains of BL-C(2). Brass was LC 69 once fired. I picked up the brass from the FMJ rounds and bagged them. While picking up the brass that had fired the boolits, I noticed two had black residue at the primer. Checked the other batch and found two more. None of the shots or recoil were out of the ordinary. Never noticed this from any loads I have fired in this rifle. The Hornady bullet was from data in the Hodgdon 2016 manual, and the boolit loading was from the Lyman 4th edition Cast Bullet Handbook. Any ideas on the cause?