Is there something I'm missing? Or by default are all the guns unlocked in a private match or something.
The guns in private match are unlocked by default. Equips some classes, exit the private match lobby, join any normal server, it should be one of your classes.
Edit: You know how in black ops, in split screen, all guns are unlocked (along with killstreaks and such). It's exactly the same. I have a P90M1 with supressor, desert eagle, bouncing betty, trophy system, scavenger pro, hardline pro, and dead silence, and I'm only level 13.