GURPS: Gun Fu gives special rules for allowing shotguns to use their effective RoF for things like Suppressing Fire and Spraying Fire as a cinematic rule. I'm inclined to permit the use of such weapon mods, known as a duckbill or gator choke, to allow the same in a realistic campaign.
I'm torn, however, between treating the attack as a narrow cone or just permitting spraying fire/etc. As a cone, based on performance in videos, I'd treat it as 2 yards wide at 1/2D range, and within 1/2 of 1/2D it's just shotgun rules as normal. As a cone, I'd also probably say you could aim it at a single target in the center and get maybe 2/3 of the shot in the center hex and 1/6 through the hexes on either side, or spin the shotgun/choke vertically to use mostly-normal shotgun rules.
Treating it as a cone, at 1/2d range you'd get 4 pellets per 12g 00 buck shell in each of the hexes if you aimed right between them, or if you aimed at a single hex you'd have 6 pellets and 1 on either side per shell.
Treating it as spraying fire would just use regular rules for lost projectiles across empty space/etc.