It appears to be 1 weapon in addition to x, so I suppose if you're firing the M2, you can fire the big gun. Or one of the MGs (which, if I remember my load out, is a 7.62mm) and the Main gun..or the M2 or even the other MG.
M240 7.62mm Machine Gun.... You might remember it as Rambo's Gun:
Currently it's in the G model, so thats:
M240G
Manufacturer: Fabrique Nationale Manufacturing, Inc.
Length: 47.5 inches (120.65 centimeters)
Weight: 24.2 pounds (10.99 kilograms)
Bore diameter: 7.62mm (.308 inches)
Maximum effective range: 1.1 miles (1.8 kilometers) on tripod mount
Maximum range: 2.31 miles (3.725 kilometers)
Rates of fire:
Cyclic: 650-950 rounds per minute
Rapid: 200 rounds per minute
Sustained: 100 rounds per minute
(Caption: A USMC demonstrates the Ablative Armor effect of USMC issued balls)