The Double-barreled Shotgun (MR43) was introduced in Patch 1.7.1.
It uses the same ammunition as the M1014 and Remington 870 (Flashlight) Shotguns. The Player may combine four Double-Barreled Shotgun magazines of the same type to one M1014 magazine. Likewise, the M1014 magazines can be split. It is very much advisable for players to combine 2rnd ammo where possible because it takes up the same amount of space as 8rnd.
Note - the double-barreled shotgun can not use the 8rnd magazines. While shotguns do not generally use magazines; in the ARMA2 engine, all guns use the magazine concept to determine how many rounds the guns can use, it doesn't track individual rounds. You must have 2rnd magazines in your inventory for this shotgun to access them, an 8rnd magazine is inaccessible to the shotgun.
Because pellets do 2000 blood damage per pellet (of which there are eight per shot,) they seem to be the preferred ammo type when it comes to PvP activity.
However its iron sights are not correctly lined up, much like the M1911, and as such is harder to aim at longer ranges.