We have the numbers for the Pool ball in the reference sticky.
We didn't have the numbers for a tennis ball in that sticky, but I just posted them.
Dimension of a Tennis ball.
...the acceptable range for ball diameter (2.575 inches to 2.700 inches respectively for a Type 2 ball). The accuracy of calibration is 1/-0.005 inches.
http://www.itftennis.com/technical/research/lab/balls/size.asp That is a pretty wide range for use in a cannon. I think it would be more useful to build a pop can cannon. Then for lower velocity use the tennis balls in that bore.
My early day experience with tennis balls using flammable gases and black powder as propellant is that without a filler, tennis balls come apart.