<b>I did not water inject them before hand for a few reasons."</b> :
1) By putting a long chamfer on the inside of your barrel, it wil allow tennis balls to be loaded very easily. I've been doing this on all of my tennis ball combustions for a long time.
This is easily accomplished with a large, <a href="http://www.demaxindustry.com/pic/product_1033003172.jpg">Half-Round Wood Rasp</a>.
If your breech loading, then I assume that you're using some type of threaded adapter? -The I.D. of the threaded portion of the adapter is slightly smaller than the I.D. of the pipe itself, so that may be yet another one of your problems.
2.) Threading, or epoxying, a basketball needle into a 3/4" PVC Female-Threaded cap works great for filling tennis balls. Simply thread the modified cap onto a 3/4" pipe nipple, and use a garden hose-3/4" adapter on the other end. (TURN THE WATER ON S-L-O-W-L-Y), unless you want to inject water underneath your skin!
Sharpening the end of the needled with a file or someting similiar will alow the needle to poke right into the ball.- Do it slowly and carefully with each ball, or the needle will undoubtedly break.
3.)It is necessary. <---Period
<b>"These next two questions are just general questions, not for school"</b>
1) Yes, mostly all brand-name tennis balls have a lot of fuzz, and are an extremely tight fit inside of 2.5" SCH40 pipe. As another suggestion, buy some "Fun Balls" at Wal-Mart.- They come in a package of 20, and are of different colors with smiley faces on them.
They're less than $5 for 20, have little fuzz, and practically load themselves.
2)Depending on your barrel length, I've found that they'll go about 1-1/2 to 2 times farther, than an empty one.