Do these achievements to unlock the corresponding effect:
Completion bonuses:
complete Story mode to unlock Magruder's seven barrel Nock gun
complete Story mode and all side missions (100% completion) to unlock Reed's armored horse and Magruder's Cannon Nock gun
69 Ferguson Rifle: defeat Hollister
Apache Shirt: complete the Hunting missions
Cavalry Sword: defeat Reed
Dual Peacemakers: defeat Hoodoo Brown
Silver Spurs: complete the Pony Express missions