Follow the Captain's orders and you'll be fine. First, things, follow your friendlies to the first mini-battle site. What you want to do is toss a grenade ahead and kill the Rebs shooting at you. On the left side down next to a dead Reb and the boxes is a Colt revolver. Pick it up, it will help you get some of the sidearm kills for another achievement. Head straight and go right, you'll find a box of ammo next to a downed wagon. Progress through the level, and if you are totally out of ammo on the revolver grab a rifle (the Henry is great). When you reach the pile of boxes with the crazy machine gun, there will be a pepperbox pistol on the ground. Grab it and get a few more kills.