The pressure is contained by the breech plug(2nd picture) just like any muzzleloader. The breech block only holds the primer in place. The strength in this system is in the barrel and breech plug not in the action itself. You are basically bypassing the action and using it simply to hold the barrel and fire the primer. This is one reason I mentioned Ballard actions earlier, you could build a more powerful muzzleloader on one that you can a cartridge rifle. I shoot 110grains of triple seven out of my roller.