Kill 500 enemies with any rifle, repeater, or shotgun in any game mode. This Trophy/Achievement takes time to earn, but it's still a time-consuming endeavor to do so. Earning it can span game modes, but if you play the single player game long enough, and wield a rifle, repeater or shotgun while doing so, you should be able to earn it during that specific experience. Remember that your numbers will carry over between the three different types of weapons.