During the campaign, you are able to pick up weapons from killed enemies. Once you kill an enemy, just walk up to their gun, and press , allowing you to pick it up. Now use it until you run out of ammo, getting as many kills as possible. It is not possible to replenish ammo for these using teammates. When you run out of ammo, you will drop the gun automatically and return to your default weapon. Now just rinse and repeat this process for a mission or two, and you should be able to get 50 kills for the achievement.