During the level where you play as Ghost and must find Soren, you can get an M240. When you are on the large walkway, save your sniper ammo. Kill the guards and keep moving until you see a woman exit from a door with an M240 and shoot some guards. Equip the sniper rifle and move fairly close. Snipe her in the head. If you kill her, she will drop the M240. Quickly grab it. If successful, you will complete the mission. If not, you will fail because you killed the woman. You will then have the weapon in the following mission.