Here in Winnipeg we have a place called Xtreme-Tactics
that is strictly CQB. Guns are limited to 350fps and 95% of people play in jeans and a tshirt, with a provided tac vest and face mask.
The general mercy rule is about 2 arm's lengths, although I think as long as you don't have anyone running a hot gun, the most damage anyone will suffer is a minor bleeder.
This is the place that got me into airsoft, and I've played games with as little as 4 people total, and as many as 12-15 per side. Its really intense, and crazy awesome action.