Unlimited Ammo
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.

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

Apache Shirt
Complete the hunting missions.

Dual Peacemakers
Defeat Hoodoo Brown

Silver Spurs
Complete the Pony Express missions.

Cavalry Sword
Defeat Reed

69 Ferguson Rifle
Defeat Hollister