Probably ten years ago I had a Bell full-face fiberglass helmet, on which the interior comfort foam had disintegrated. I wondered whether a helmet would shatter if it were hit hard enough to actually penetrate it. I put the helmet on an upright piece of firewood, and shot at it. The wood was to stop any bullets which penetrated, so as to give me more places to shoot. A .22LR from a 4" revolver did not penetrate, but did chip the finish. The .22 did not penetrate the face shield, either, but made a mighty ugly permanent dent. A mildly-loaded .38 Special did not penetrate the helmet, but punctured the face shield. The .357 Mag went through the helmet, missed the wood, and exited the other side of the helmet. Aside from the obvious damage (the bullet holes) and a damaged area no more than half the diameter of the holes beside the holes, I could see no damage to the fiberglass. There was no visible cracking and no apparent damage to the finish anywhere else on the helmet. I have not yet had a sacrificial ABS helmet to repeat the experiment. My purpose in the experiment was to see for myself whether a helmet seemed likely to be damaged by the classic "Oops! It rolled off the seat onto the driveway." I concluded that besides the minor scratches in the finish, there was unlikely to be any damage. Standard disclaimer - your results may vary.