Pepperbox is the title of a group of pistols in Borderlands: The Pre-Sequel. They are manufactured exclusively by Jakobs and features a Vladof barrel. It can be obtained at random through any appropriate loot container (The Nexus arena works well for this) or through grinding glitch weapons at random.
This gun cannot be obtained by legitimate means in Borderlands 2. Pepperbox can only be obtained through third party programs or by trading with someone who hacked it into their game.
The Pepperbox was meant to be the title of any Jakobs weapon that used the Vladof barrel, it was cut from Borderlands 2 presumably due to having an increased fire rate on a Jakobs gun to be pointless, as players could not achieve higher fire rate physically on most Jakobs pistols anyway.