The Roundhouse games in Monterey can be fun. The public games usually have 200+ people out and with a field that is a combination of indoor and outdoor. Like others have said, it's a fun way to shoot people without killing someone. The only thing is that it can be expensive to start; gear is not cheap.

http://www.cqbchallenge.com/html/cqb_challenge.html. For a civilian, this is as close to the real thing as it gets. I know a lot of the trainers, and the course is a boiled down version of actual CQB/MOUNT training.