I'm fairly new to the game, but with my loooong experience with FPS's, its all pretty much the same. Just simply stay down, and wait. In real war, the only person stupid enough to run out blaring a MG, would be a marine with Cerebal Palsy. Don't be stupid. Stay down, pop up and down, burst fire dont't spray, recoil KILLS you in battle, because whilst your shooting up, your getting drilled in the chest by a well aimed bullet, corners are your BEST FRIENDS, use them to have advantage on cappers from the other team, stay secluded. ALWAYS use iron sites unless in close combat, then spray your ass off.
That about wraps it up, hope I helped.