As it has happen way times here they don't normally allow this type of thread to go on to long, but since it open for now.... I have shot paintballs, airsoft, and BBs at my fiberglass costumes and nothing has happened(alittle paint chipped off the BB one but that was it)but its a special layup not just your weekend warrior job,I have shot Cast plastics ones as well and most have at leasted cracked the BB shattered the Alumalite one I tried it was not a pretty site and the Paintball cracked a Roto Cast one I did for a client a while back but it was pretty close to the helmet and it a pro gun.
So unless You really really know what your doing with fiberglass I would not go running around the woods in your Halo costume shooting each other thinking a little paper and bondo is going to stop anything cause you'll need a trip to the hospital or at least your friends will have a good youtube vid at your expense...
Just don't do it......