Barrels
Some machine gun levels feature yellow barrels, usually with enemies nearby. Shoot those barrels, to detonate them and kill nearby enemies. This will result in bonus cash.
Various Gameplay Tips
Enemies figure out where you are once you fire after dropping into a zone, but you can use some quiet time ahead of that to conduct a little recon and give yourself an advantage from the start.
While you are checking the area for enemies, before you ever fire a first shot, make sure to tag them by letting them pass through your crosshairs. That will make them easier to spot when the action gets frantic.
Aim for the head. You won't have to worry about body armor soaking up the damage, and you'll generate extra money.
Whenever you can, upgrade your sniper rifle. Once you do so enough, you'll earn the silencer so that you can shoot without enemies figuring out where you are.
If you see yellow barrels, shoot them as enemies are nearby. The explosion nets you a bunch of points while also taking out your foes.