"It was dark. The car pulled along side and some dude leaned out with what appeared to be a suppressed rifle I decided as I heard the sound and felt the impacts. Fearing for my life, I drew and fired on the fleeing gunman as I believed they were still shooting at me."
It would be a good comment, maybe, but no doubt there would still be some legal hassles. The real problem would come from knowing they were shooting paintballs and admitting it. No doubt there is plenty of information to substantiate a claim of fear of bodily harm, serious bodily harm. Specifically, you can do seaches and find stats for the types of unprotected eye injuries from paintball projectiles and that will usually be from nominal pressures. Some paintball guns can have their pressures turned up quite a bit above competition levels (which I understand are something like 300 fps or less) with some approaching 500 fps.
You could argue that you had no idea that it was paintball at first and by the amount of pain you experienced, you felt that you were not being shot by normal competition velocities as you experienced significant pain you felt was serious injury.
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You could be justified in firing to stop an attack on you, but would likely have a lot of trouble in firing on suspects fleeing in a vehicle after the actual attack had stopped. You would have a tremendous amount of trouble justifying shooting back at fleeing suspects in a car if you weren't hit, I would think. You might have been in fear for your life and the punks might have deserved it, but especially if they didn't hit you and you did hit them, there would be complications and in the short run the criminal complications for you might work out, but the civil ones may be a really long and painful experience for you.
If you know it was paintball and are not shooting to stop an attack on you, shooting back is not going to be very beneficial to you in the long run.
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added comment...
Another thought is that paintball guns may not sound like firearms, but they may sound like air rifles. There are some really amazing air rifles out there. This is one of them and it can shoot a .22 caliber projectile at over 1000 fps. Even wilder is that there is even a suppressed version...
http://www.webcom.com/airguns/AF_TalonSS.html
This gun would literally have the potential of being more lethal than some firearms.
I originally marked this site after somebody had posted that they found a great in-city varmit gun that was perfectly legal in places where shooting an air gun is legal, but has the power, accuracy, and extended range more in line with a small .22 rifle.