Well, the thing is, anytime you've got mass and velocity, you've got energy -- so the risk of "hurting someone" is there. A tennis ball is a good example; it's light and not overly hard, but when you start putting enough energy behind it to fly straight for more than a few meters, it'll smart if you get hit by it.
If you're going to do a non-BB-launching projectile, you're maybe better off with a very lightweight (NERF-ish) projectile that'll emit a small bang on impact (put a toy cap in it).
I can see players having a problem with being covered in flour, too; you'll look white for the rest of the game. A cloud of flour also poses a flash hazard (risk of explosion) if exposed to an open flame or similar ignition source (no joke).
As for indirect fire projectiles picking up velocity "on the way down", first this is limited by terminal velocity, and secondly something that has enough energy (velocity) to escape gravity (i.e., fly in a straight line) has more energy than if it were being pulled down by gravity alone.