I’m working on a old model12 pump .22 rifle. It all seems to work fine except the action does in lock after it is pump once. You can keep pumping. What locks the pump after the firing pin is loaded. I have not tried to shoot the gun. I would not think a .22 would be able to push back the reviver. There is some resistance but I can keep pumping. My question is what locks the receiver after the first pump?