I got shot two weeks ago from my KJW M700 (thanks Testie, never been hit that hard before) from maybe 100ft (I was in the tower and hunkered down pretty good trying to engage attackers with my M24), right in the forehead with a 0.30g BB. This gun at one point chronied around 550fps in hot weather (+35C) using propane, previous times I've chronied it this year it sat around 450fps roughly (+/-30fps). The day I got shot, it was what, +18C out or something?
Anyways, it hit hard, left a good welt with a bit of blood, I liked showing it off over the next few days............... took about a week for my little scab to finally go away............ today, you can still see a small light purple area from the impact. AND, I don't bruise very easily at all!!!
So ya, they carry decent kinetic energy, is why they are more accurate and have farther range than lighter BBs.
TIP: Use the same BBs in your GBB as you do your sniper rifle, they work amazing for range and brush penetration!!!