You also have to remember that unlike real bullets, one BB is unlikely to get its 'kill' across.
This is to say, one bullet will have an immediate effect on your target either killing him/her outright or at least effecting tremendous physical trauma.
One BB, on the other hand, is unlikely to be felt or even noticed. You'll need several BBs on target to 'kill' your opponent.
I'm sure we've all had this happen to us before: lighting up a guy with a short burst, he doesn't feel it, and you have to follow that up with two or three more bursts before he'll call out. BBs just don't carry the same effect.
In a similar situation with bullets, however, you can be assured that if your rounds are on target, your opponent will be done after the first or second rounds.
This is not to justify the use of ridiculous highcaps, but rather to place into context the balance between being 'realistic' and being realistic about BB terminal ballistics.
Contras A-06: F**k it, we'll do it live.