I just bought two barrels today. One 7ft long copper, 1/2 pipe, and one 5ft galvanized steel 1/2. They both have different inner diameters, for the two ammo types I will use.
Wait a sec, you're using 1/2" diameter barrels on a hybrid with a 2" union? I'd suggest building a different cannon for barrels that size and use this hybrid as a big-bore launcher.