Early on I made a similar incorrect conclusion, that machine guns are inferior to rocket launchers. Do yourself a favor and explore a bit to see where machine guns shine, especially when you blip the trigger and head shot mobs for easy kills. If you're more in the spray and play group, then yes, machine guns might seem a bit useless, but try doing patrol bounties using a machine gun primarily and conserve ammo. A MG like SGA becomes a blast. I bet you'll start to use them more often!