I'm a sponsor and we have surplus military barrels a lot of folks like to use, since they are about the strongest tube you can get. We have 40MM Army "Duster" barrel sections, 40" long, weigh 90 lbs, 12-groove rifling, twist adequate for black powder shooting with projectiles made out of any material that can deform just enough to catch the rifling. 40" section is $400. FOB zip 22150, normally costs $80. or $60. E. of Mississippi, or pick them up in Alexandria, VA.
We also have original 7-pounder muzzle-loadiing rifles, steel, 1880's, priced slightly higher. And often we have other interesting items. Oh, we just got in some massive 105mm rifled tank gun barrels, 4-foot sections, weigh several hundred pounds each, probably average 2 to 4 inch wall thickness, nice for a rifled howitzer! No pricing set up yet, but working on it.