The Facts of Impact
Paintballs are large projectiles, .68 caliber on average, and they’re usually moving at around 300fps. This makes for a substantial impact, but the nature of the plastic shell means it shatters on impact and spreads the gooey goodness that signifies a hit around so it’s not equivalent to getting hit with a rock or anything.
A hit on bare skin will generally leave you with a welt, especially at closer ranges. Actual bruising may happen with extremely short range hits, but this is pretty uncommon if you’re playing on a regulated field since the offender will undoubtedly be removed in short order.
Of course the angle of the hit, your distance from the enemy’s gun, and the area you take a ball in are all huge factors in how much it hurts.
An arced impact in the chest from across the field isn’t going to cause much pain, but a short range burst that peppers an unprotected hand and forearm will have you cursing and wondering why you picked up the sport in the first place.
Without protection there will definitely be a bit of impact and pain. The associated damage shouldn’t be heavy, you won’t be bleeding or anything from a regulated paintball hit, but it can leave some unsightly welts and bruising if you get shot up at close range.
The level of pain you feel subjectively will vary from person to person, some people have rather sensitive nerves and might find that they need a ton of extra protection to avoid having their day ruined. Others will be able to gleefully take multiple hits on unprotected areas of their body and run around the parking lot showing off their welts to others after the game is over.
Most of us are in the middle. It hurts enough you won’t like getting hit, but it’s definitely not enough to worry us during the match.