Bullets push deeper into the case for generally one of two reasons. Case has a cracked mouth or lack of crimp. This is less likely with BP ammo because the case is full of powder and/or wad/filler so the bullet can not push deeper. With your loaded ammo, if you have sufficient crimp, you should not be able to twist the bullet in the case and pushing the bullet end of the round against a hard object should not push the bullet deeper into the case. If your bullets are .427 then perhaps you are over-expanding the mouth. Some dies are setup for .429 bullets.