Ok, since everyone seems to be really clueless when it comes to CPS tubing I'll explain what I know that applies to Nerf.
First, you never, ever attach any tube to straight PVC, or even a male threaded adaptor. Doing so is like asking to have your gun fall apart in your hands. You should only use barbed fittings, they have an end for tubing and another threaded male end that will screw into your female threads. You also are required to use tubing clamps, because if you don't then after one or two pumps your chamber will pop off. If they aren't on tight enough, especially in a water gun, the stuff could pop off at a very high velocity, and you don't want to get hit by that.
The CPS tubing should be smaller than the barb you are using. That will let you stretch the smaller one over the larger tube. In my gun I didn't do that, but that was because I made it out of leftover parts.
The site for all your CPS tubing needs is
http://www.mcmaster.com. Go there and search for latex rubber tubing, what it is actually called. The thing will list all the info you should need to know. I think the PSI actually is the amount of pressure it takes for the rubber to expand. Just get the thickest you can get, and then get something to put over that, something where the ID is slightly larger (not the same) and the OD of the smaller tube. That will let you make it thicker, and not spend hours trying to get something with and ID the same as the OD on. Don't make that mistake.
The Elongation of the tubing is 850%, which means that if you multiply the ID by 8.5 and then add the thickness of the tubing times 2 you'll get about the size of the tubing when expanded. For multiple tubes, just add up the thicknesses. Don't make any jokes about how you can order this stuff in black and how it gets 8.5 times longer either, that is just silly comparing this stuff to that.
http://www.sscentral...to/DSCN0102.JPGThere is my gun so far, if you want more info about it I'll make a new topic (it is far from finished). You can see the CPS tubing at the bottom, both ends sealed, one with a barbed fitting and one with a big screw because I ran out of barbed fittings and threaded endcaps. ou can't see it, but the clamps literally are cutting into the tubing, when it gets to that point you should stop. I've considered covering the ends with electrical tape and then putting the clamps on so that it's cutting into the tape instead. You should house this thing is some PVC about the same ID as the OD of the tubing. This stuff gets about 3 inches and 1 sixteenth big, which is close enough to 3 inches that I'd use some of the 3 inch PVC I have so much of. If it's like 3 1/2 inches then you should use 4 inch because it's not close and it's better to have more air.
If you've got any more questions about CPS systems just ask, I probably know it.