''Marker'' Tennis balls

[Berkut]

Myself being a milsim airsoft player, I would hate someone shooting paint-anything around. You can throw your camies (camo uniform for those that don't know) into the washing machine, but its hard to get paint, even water washable, out of MOLLE webbing and pouches.
My area's rules are that ammunition is a nerf howler football, and in order to use any home-built weapon, it must be inspected by an admin, and you must be willing to take a hit from it. They'll do it too, I've seen it.
I wouldn't worry about marking the hit, people do call kills when a couple howlers come screaming in.

[spudamine]

As others have said, using a tennis ball is a bad idea.
these are shots from my tennis ball cannon, 1st shot is 65psi burst disk, 2nd shot is combustion:

[youtube][/youtube]

Your cannon might not be as powerful as this but these are just empty tennis balls, would you stand it front of that?