In Red Dead Redemption for PS3 and Xbox 360, you can unlock one of the most powerful rifles in the game called the Buffalo Rifle by completing one of the Ambient Challenges. It has the highest power value, but is slow to reload and can only holds one shot at a time so you have to make your shots count.
Don’t forget to use Dead Eye to instantly reload your weapon, especially for slow loading weapons like this one.
In order to unlock the Buffalo Rifle, you simply need to complete Master Hunter rank 5, then you can buy it from any gun shop.