Look into some XXX heavywall seamless pipe. It will be expensive but you wont have to worry about blowing it up unless you do something stupid. Mine is XXX 1 inch @ four feet long. It eats about 400 grains of BLACK POWDER (this does not refer to a color, it is the composition), and shoots a 2800 grain slug. I threaded the breach end with a fine thread 1 1/8" tap about 2 inches deep. Drilled and tapped for a musket nipple and made a slap hammer activated by at least 50 feet of line. This is not a toy though the ATF considers any front loading blackpowder charged gun to be a toy no matter the bore size.
Good luck. Don't hesitate to ask questions. If I don't know, I will tell you I don't know.