I talked with Joel Brown of Hern Iron Works today about their barrel liners.
All of Herns full scale guns are made with a 1/2 inch thick wall seamless tubing with a pressed in and welded breech plug that has a radius face.
Smaller than full scale cannons have a 1/4 inch thick wall tubing that has a no bead welded seam. The breech plug is flat faced, pressed in and welded. If you desire a seamless tube in the smaller guns inquire upon ordering.