Based on the AA-12, the Automatic Shotgun is available in two forms: its more conventional form functions like a regular shotgun that fires buckshot, while the second, more devastating form, known as the Explosive Shotgun, is capable of firing "FRAG-12" explosive rounds that can inflict heavy damage on vehicles. Regular vehicles can only survive two direct hits from an Explosive Shotgun before catching fire. The rounds have massive concussive force, easily denting vehicles. The Automatic Shotgun can kill an enemy in one shot up close or strike an enemy down from a distance.
The regular Automatic Shotgun is a separate weapon from the Explosive Shotgun; this means that in order to switch ammunition, the player must discard their Automatic/Explosive Shotgun and lose their ammunition reserve (unless the weapon is picked up off the ground). Regular Automatic Shotguns are distinguished by red shotgun shells in the loading chamber, while Explosive Shotguns feature green shells.
Differences may also be noted in their ammunition capacities, while both variants are capable of holding 20 standard or explosive shells per magazine, the Automatic Shotgun can only hold 230 regular shells in reserve (the same carrying capacity as The Lost and Damned's Assault Shotgun, which can hold 8 rounds in the magazine and 242 rounds in reserve), while the explosive variant can hold up to 580 rounds in reserve. In terms of rate-of-fire, both variants fare identically, although neither of the variants fire fully automatic like the shotgun's real world counterpart or the Assault Shotgun (probably for balancing reasons). Rather, they behave like semi-automatics in a similar fashion to the Combat Shotgun in GTA IV.
The range on both variants in impressive for a shotgun. The buckshot variant has similar range to the Pistol, and will never take more than three shots to score a kill. The explosive shotgun has range outclassing SMGs, and will usually kill in just one shot, but occasionally it may require two shots, but never more.
- The Explosive Variant is received from Yusuf Amir in the mission "High Dive" and again in "For the Man Who Has Everything". After "High Dive", the player can purchase explosive shells from Armando.
- Both variants of the Automatic Shotgun are also available in Multiplayer.
- Explosive variant spawns in Safehouse after completing 30 Drug Wars and "High Dive".
- Luis Lopez asks Yusuf Amir for some more explosive shells in the mission For the Man Who Has Everything because Luis finds them impressive.
- The real-life counterpart of this gun, the AA-12, has been featured in Future Weapons, a program from Discovery Channel, as well as one of the videos of popular YouTube channel, FPS Russia.
- If the player catches fire or falls while holding the Automatic Shotgun and fires it, the weapon will become fully automatic. Though this is a glitch, it is evidence that it was originally fully automatic. The rate of fire was possibly made slower to balance it out on multiplayer.
- Since it is a shotgun, it has the cocking animation of the Pump-Action Shotgun when blindfiring, which is rather unrealistic as the shotgun has no pump.
- The green color of the explosive shells suggests that they are deer slugs, which would be the logical choice for an explosive shotgun shell, as explosive buckshot would not cause nearly as much damage.