Because of its weight, you can't fire it while moving. FA's only machinegun is the M-60. It has horrible accuracy unless you're lying down with it, but its 100 round box of 7.62mm ammunition can rip people in half. Good for breaking legs and laying down supressive fire. The ultimate support weapon.
Claymores are nasty weapons that almost always kill someone when detonated at their feet.
To place a claymore, hit the fire button. To detonate it, hit the fire button again. Hitting secondary fire when its already been placed sets it into trip mode, and anyone silly enough to walk over it gets blown into meaty giblets. Your standard high-explosive grenade. Lob one of these into a room, and people run for their lives.