I have been wanting to craft a very crude primer fired black powder firearm for a while. Getting a new job at a hardware store has only furthered my desire to craft a firearm. I have seen people using black iron piping to make slamfire shotguns in the past, which is why it's my initial candidate for a barrel. My first question would be if any size of black iron can handle the pressure of black powder. I plan on getting around 10 inches of whatever size fits ready made led bullets (smooth bore shooting a ball) and putting a cap nut on the other end. For a firing mechanism I was going to try and make a hammer that hits a primer, as opposed to an entire flintlock.
If black iron can handle the pressure what size would be best? If it can't are there any "hardware store" alternatives that don't require heat treatment?