There's an exploit in the first Chapter 1 The Pyrenees. Right before Gabriel Nowak defuses the bomb (and gets shot), set a C4 charge on a pillar where the left gun turret will deploy.

When you order Nowak to "Team Defuse Bomb", detonate the C4 to kill the enemies past the shutter; this delays Nowak being taken down (which in turn triggers more enemies to assault the room, as well as Alpha Team to come in, thus ending the stage).

As soon as you detonate the C4 (and Nowak is working on the bomb), one enemy will continuously spawn on the balcony above where Bishop and the team entered the bomb room. An emplaced machinegun will mark where that enemy will run to. Simply unload your weapon on this enemy and a replacement will spawn with no limit.

If you pay attention to your ACES menu, you can determine which points you earn. Step close (right under the turret) to get CQB points. Shoot the enemy in the head for Marksmanship points. Allow the enemy to take up the gun turret before neutralizing him for Assault Points (do not shoot him in the head or it counts as Marksmanship).

Finally, if you need to change weapons, you can always get the checkpoint before the bomb, quit to the main menu, re-outfit your PEC, then continue story mode. There are also two supply bins at the beginning of this area (the entrance to the plaza) where you can change (and refill) your weapons and gear.