I've seen guys bleed quite severely with stock Maruis shooting sub-300 fps. All the teeth I've seen knocked out were with stock guns (onoly 2 or 3 times). And we have been using gas M700's, PTWs and heavily upgraded AEG (450 fps) for several years now. And the most damage was done with the stock guns.
Is the hot gun or the inexperienced user? Smart money is on the user every time.
And I don't play mercy. Mercy is a sketchy rule at best, and becomes an issue of "he said/he said" over who calls it first, how close you were, etc. A BB doesn't lie. If you're close enough to call mercy, you're close enough for a 1-2 shot in the ass.
Besides, I'm an old, out-of-shape, noisy guy who doesn't crawl. If you let me sneak up on you, you deserve a shot in the ass.