How do i get the golden mauser pistol in red dead redmption

single player- For sale at the Blackwater Gunsmith ($800)

multiplayer- you can either level up or it can be found in the gang hideout in twin rocks (defeat the gang then its in one of the houses)

Sharkboy67 here... the guy is right, you have to buy it at the Blckwater Gunsmith for $800, in order to get the golden mauser pistol you must purchase the Golden Guns/ Weapon pack in the game marketplace under Red Dead Redemption... it's only 80 microsoft gamer points.