Okay, so I'm a contractor, so most of my combat would take place within 100m since I would be providing security for rich folk, important folk, and convoys. That said, I would want something compact that doesn't need to be accurate past 300m. I would probably take either an MP5 (yeah yeah, submachine gun) or else a Rock River Arms AR-15 style rifle (forward pistol grip, collapsing stock). I'd carry standard 30 round mags and a 100 round C-Mag.
**Edit** I'm not a contractor. By saying I was a contractor was putting myself in that hypothetical situation. Sorry for the miscommunication.