For this achievement you want to load up the game and on every map there’s a different spot the gun will spawn at. So once you find it you wanna make sure it has all 5 just in case you miss a shot. You are gonna want to aim for the head because it will be quicker. So just kill 2 people and the achievement will pop up. If you run out of ammo you will have to get a new one. If you want full ammo just wait for the gun to grow fully.