.510 Whisper. Nominal load is 27 grains of H110 for 1050 fps with a 750 grain bullet. Had one round that the Encore action refused to close on. Put the round in the dud box. A year later I pulled the bullet and found a double charge in the case. I think the action would have held together had I fired the round, but it might have bulged the chamber and cracked the frame.