Smaller one and just seat it until it engraves completly through the base or just shy of it. If wanting to go shy just seat a bullet, pop just a primered case, wait about fifteen seconds and then open the breech. If sealed the gas will eject the case and don't stand behind it. If not sealed just push it in past the base and I am very confident that it then will. You might have to check the diameter of your seater plug not being larger than the bore to do so. If so you only have to turn down just enough of the nose length to be able to accomplish seating it that deep and not the whole thing as you still want the bullet to be supported at the base and staying central to the bore when seated.

JLouis