I think making a helmet for paintball sound like fun! From my experience with paintball (24 years), I will say be careful with the visor. Using a high impact plastic for the helmet will be more than strong enough. Just make sure that it is at least 3/16 inch thick. Now the visor is very important, taking a shot to the head is no big deal, but if your visor fails you're looking at losing an eye. The goggles that I use have a dual pane polycarbonate lens, I have been shot dead in the face from inches away and not a scratch. So I would recommend trying to fit a set of "goggle lens" into the helmet. You will also want it to seal up around your eyes because shell shrapnel from a paintball can still make it up to your eye. So heck ya, build it, but remember safety first. 300 fps is roughly 280 mph