My marble machine gun was giving me all sorts of trouble. The paddle just stopped working periodically and finally I just got tired of it. I couldn't enjoy shooting it.
So, replaced the pop-off with a 1/4" mnpt check valve and male quick disconnect, and then removed the entire tee loading system.
I now have two barrels I can use for it. One is the original 1/2" x 24" steel pipe for marbles, which I just drilled and tapped for a small screw at the back for a ball stop.
The other barrel is a length of .68" ID hydraulic aluminum tubing. Perfect paintball barrel. Even beats all that 3/4" CPVC or 3/4" SCH-160 or whatever is supposed to be "great". This barrel is 40" long and is connected to the gun with a 3/4" compression adapter.
The valve is just your run-of-the-mill 1/2" quick-exhaust valve.
I can't wait to do some testing with the paintballs, and I'm going to pick up a bucket or so sometime soon because I need paint for playing anyway.
Here's some pictures.