Yes...it is possible.
I had a barrel that I was shooting tennis balls from. I was shooting them from my 'BMFG 1693' combustion cannon. Tennis balls can be found in quantity at 'Play it Again Sports'.
You need to use 2.5-inch diameter PVC pipe (gray) for the barrel and adapt it to fit standard Schedule 40 fittings (lots of Dremel work).
Tennis balls don't shoot very well. They create a lot of resistance as they travel down the barrel so they don't go very far...funny, but not far. If you had a pneumatic cannon and you wanted to shoot tennis balls for the dog to fetch, it would be okay. But, if you want to shoot tennis balls far as a substitute for potatoes, it won't work. It is simply and inefficient projectile.
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I know a little something about size, volume, and efficiency. I recently converted my tennis ball 2.5 inch barrel to shoot potatoes and it is averaging 700 to 800+ yards from my BMFG 1693!!