The Sub Machine Gun may possibly be the replacement for your Assault Rifle from Halo: Combat Evolved. Of course, the only way it could be a replacement was if it was better - and it is. You can carry dual SMGs, and put twice as much Covenant-killing lead in the air at one time. Of course, the SMG has a wide spread, and you're not guaranteed to hit something if you just fire clip after clip. As with the Assualt Rifle, it will probably be wise to fire in small, controlled bursts.