TamponCrafts has published a HOWTO for building a paintball-style gun for firing tampons:
Inspired by marshmallow shooters, this air-powered tampon gun turns your feminine hygiene products into high-flying projectiles. Have a shootout between rival tampon brands, or use it as a fun alternative to paintball. The tampon shooter has a range of 10 to 20 feet depending on your ammo and lung capacity. The matching bandolier lets you carry a full "clip" (i.e., box) of 20 tampons, so you'll never be caught short in the heat of battle.