I have a 3' piece of 4" ID, 5" OD steel pipe, with a 2" solid steel plug machined for, and (carefully) welded into one end. I think it might also have a 1/8" hole drilled into the end with the plug in it, but I'd never think of making a cannon. ;-)
I’ve heard some rumours about people with similar pipes making model rocket bazookas. Of course, I’d think someone doing that sort of thing would probably want to wear a motorcycle helmet with a full face shield and thick leather gloves, but of course, I’d have no way of knowing for sure.