Successfully complete story mode to unlock Magruder's seven barrel Nock gun.
Successfully complete story mode and all side missions (100% completion) to unlock Reed's armored horse and Magruder's Cannon Nock gun.
.69 Ferguson Rifle:
Successfully complete the Hunting missions.
Successfully complete the Pony Express missions.
Take a barrel of TNT from the Badlands mine. Find the abandoned house in the Badlands where the buffalo graze. Go to the back of the house on the outside. Face the chimney (looking west) and place the TNT barrel on the left side of the chimney. Blow up the TNT barrel to open a hole in the ground with unlimited ammunition for all weapons.