You should be pretty close to this by the time you finish Veteran Mode. If you still need to fire out more rounds, simply select the LMG in your armory and spray away!
Another good way of burning through some rounds is to load up Mission 7. This is the mission where you will be in control of the jet and have to destroy enemy turrets on the ships. At the very beginning of the mission where you need to destroy the turrets on the first ship, simply destroy them but don't shoot at the other objects on the ship. The jet will sway back and forth, so aim your cross-hair up in the sky and fire away.
You can check your statistics on the Main Menu of the game by pressing .