A double-barrelled gun (of any sort) is treated exactly like a normal version with one exception. Any enemy is hit by two blasts rather than one (ie, a pistol causes two S4 hits rather than one for each successful hit). However, each barrel takes a full turn to reload (although if you reload only one barrel you can fire it like a normal pistol/rifle). If you own a brace of double-barreled pistols you may fire them every other round (rather than every round like a normal brace).
Lore
Knowing Ostlanders’ penchant for impressive weaponry (and ready willingness to spend excessive amounts of money) a weaponsmith from Hochland decided to weld two barrels together on a pistol and sell it for twice the price. The warband was so impressed with their new weapon that they asked him to do the same to their hunting rifle. Since then, the weaponsmith has been flooded with orders from some of the most powerful warbands in Mordheim.