SpudFiles

Atempting to make a low powered tennis ball launcher, couldn't find a suitable piece of pipe for the barrel so made my own from flat piece of stainless, not perfectly round and has a bit of a seam but I think it will do the job.

I think it will too. For whatever it's worth, I say mount the barrel with the seam at the top and pray for a backspin.

Very cool. I made a tennis ball cannon in high school from a rolled and welded sheet like that. It worked perfectly and I bet yours will too.

Sorry for late response, just a heads up.

2-3/4" exhaust pipe from any auto parts store has a Inside Diameter of 2-5/8ths of a inch. ($30 to $50 for a ten foot piece.)
A tennis ball fits perfectly into them.