I have to agree with those against powder of any sort. I'd stick to the honour system, and something a little more solid but non-painful when it hits you (read: tennis ball, nerf howler, something like that). Something to keep in mind, a tennis ball will only fly about 80 or so yards. No matter how hard you throw it. We discovered this when we built our catapult. We used tennisballs and golf balls, we had it heaving a tennis ball about 80 yards, tried a golf ball, 120 yards. Then we cranked up the power and had the golf ball going about 200 yards, tennis ball still only went 80 yards.
WRT howlers being difficult to replace, you could always just have the person who gets "killed" by the mortar fire pick it up, and they make noise, so you could always try and track them down later. Paint them hunter orange, they'll be pretty visable at that point, and they sound more realistic as mortar rounds anyway. Plus they look kinda like'em too
