NEVER weld up a pipe for a black powder cannon, I have seen many that had the back end blown of, Im a certified welder , and would never attempt to do that
Thunder mugs such as this with a thick base using schedule 80 pipe are pretty safe, assuming an experienced welder does the welding...sorta like this:
http://www.cannonfuse.com/store/pc/-NEW-5-Tall-Signal-Cannon-p11944.htm I was certified welder years back (not sure of expiration dates?) and I'd be very leary of welding a plug on a shotgun barrel...I'd go for the wind chime option myself. Shaft/hardened steel that is turned & bore-drilled leaving a thick sidewall is what you really want for a small caliber signal cannon.
Having said that....do check your local/state laws regarding possession and use of such "signal cannons". Not saying that I have ever had possession of such an item...but, I do have some "knowledge" of things of this nature. The site above is a good source for fuses and using the guide below (I am NOT the author) is just a simple reference point for loads that should never be exceeded. Toilet paper or paper towel material is good for wadding. Never, ever use any kind of projectile in a "signal cannon". Above all, don't be an idiot when handling such things and use common sense. The internet & youtube is your friend for expanding your knowledge of such things and entertainment for seeing the gene-pool nearly reduced.
