I need a recommendation for a tennis ball gun...

[Shango]

...What schedule, diameter PVC shud I use?

[potatoflinger]

Well, the first thing you should do is check out the Wiki (there's a tab for it at the top of the page). That will show you a lot about spud guns.
The best type of pipe to use for a tennis ball barrel is 2.5" pipe. It is hard to find 2.5" pvc, but you can use 2.5" electrical conduit.

[base45]

yea im pretty sure the easiest way to go is using 2.5" pvc which is hard to find( cant get it at homedepot, or lowes) but try some pluming store in your area

[BC Pneumatics]

http://www.bcarms.com/product_info.php? ... ucts_id=33

That would be the right size for a tennis ball barrel.
There are couplers, bushings, valves, etc on that site as well, if you were wondering about those things too.

[Shango]

Thanx a lot folks!