Why is everyone going on about epoxy? The OP needs to take a look at the real problem: His threads.

Properly tapped NPT threads should hold the fitting securely without the need for any adhesive. You either drilled the hole too large, threaded the tap through too far, or used the wrong size tap ...

Sorry about that - guess I should've included some more details.

The epoxy was a two-part plastic epoxy I picked up at Home Depot, but I don't have the container anymore, so I don't know the name. I had let it cure for @ 30 hours when I handled it. didn't put much pressure on the trigger and it ...

I just finished(maybe) my first pneumatic tennis ball launcher - a simple, over-under design with a modified Rainbird 100-DV valve and blowgun trigger. I'm having a problem with the trigger, though. I drilled and tapped a 1/8 NPT hole for the trigger, and used a plastic epoxy to attach a brass ...

A source for fittings is a good electrical supply place. Sched 40 non metallic conduit (PVC) has the same dimensions as the water pipe cousin. Non Metallic electrical conduit is grey instead of white.
http://s7.cdn.hardwareandtools.net/is/image/HardwareandTools/034481061083?wid=200&hei=200

Didn ...

If you're still looking for 2.5" fittings, try to look up a Local plumbing supply place. If you can't find them there then order from flexpvc.com. They have great prices and a massive selection. If you manage to need something they don't have (i did) then email them and they will probably have it ...

Can we have a screenshot?

I haven't actually run numbers to get an accurate idea of "dead space", but here is an example with (very) rough estimates. The only variable I changed was the amount of "dead space".

3 cubic inches of 'dead space" - valve on barrel end yields 152 fps muzzle velocity ...

If super performance is not a concern and very compact size is important the short chamber and valve together work well as shown in the original post. As JSR mentioned, the valve at the breech of the barrel will perform better, but it will considerably alter the overall length.
Use GGDT to model ...

Schedule 40 is a tight fit on a tennis ball. If you get the thinwall pipe instead, you won't need a ram rod to load the barrel. You can see the thin wall pipe loading in this video.
http://www.youtube.com/user/Technician1002#p/u/6/s0l5t7D0FDM
Schedule 40 pipe will require a ram rod to load it ...

4) I already bought some Schedule 40 pipe, including hard-to-find 2.5 in. for the tennis ball barrel. The plumbing guy said all the fittings were Schedule 40, which meant they were also pressure rated, but there's nothing stamped on them. Is this true? Are all Schedule 40 fittings pressure rated ...

I'm looking at building my first pneumatic cannon and would greatly appreciate any advice. I'd like to keep it very simple for a first build, and I'd also like to keep the size and weight on the small side to make it easier for the kids to use. The most likely use would be to launch tennis balls ...