I've recently decided that it's high-time I build myself a shotgun. I'm fairly deadset on building either a break action or falling block, just for simplicity's sake. 12 GA of course. I believe my biggest obstacle is going to be finding a proper piece of steel for the chamber and breach block. That being said, what sort of steel do I need to use, 4140? How much chamber pressure can a 3 1/2" magnum shell generate? Additionally, how much would it cost to bring my piece of steel to a machine shop and have them bore it out and trim it down to a set of specs I give them? I sure as heck don't own a lathe. Thanks in advance.