Take a barrel of TNT from the Badlands mine and place it at the north end of the old abandoned house's chimney. When it explodes it will rip a hole in the ground with unlimited ammunition.

Finish the game 100% (this includes side stories) and then go to the Macgruder mine to get the horse and shotgun.

Complete the hunting missions.

Defeat Hoodoo Brown

Complete the Pony Express missions.

Defeat Reed

Defeat Hollister

Successfully complete Story Mode