The weapon wheel sorts weapons (clockwise, from the top) by: When dueling or in certain cutscenes, John and Jack will use whichever pistol or revolver is currently equipped.
Only one side-arm and one long-arm (the Sawed-off Shotgun counts as a long-arm) are visible on Marston at any one time. The player can switch between guns of the same slot.
Each weapon occupies a slot on the weapon inventory wheel. The player can carry every weapon that is acquired in inventory, but only eight are active for selection at once.
