@Nice_Boat: shotguns actually have huge spread, especially for sawn-off shotguns and hunting shotguns. You might wanna say that hunting shotguns (the ones with double-barrel) don't have spread, but you are wrong, because mostly slug rounds are used instead of shells.
I'd agree as to sawn-offs to a degree. Hunting shotguns... well, it largely depends on the choke and the ammo used, but with the point of aim being roughly the middle of a human torso you'd easily close to 100% hits with 00 buckshot (and shooting humans with anything aside from 00 and slugs is not effective) at 50 meters, with 40 being the effective range of Mossberg 500 series with ~20 inch barrels. Of course at the most extreme distance you could miss at the center of the target's mass and have something like half of the pellets fly by, but it's really so marginal it's not even worth modeling in game, and if the spread becomes too large and not enough pellets hit, the damage drops considerably.
Of course you could say that people don't stand upright facing the enemy in a firefight and tend to drop their stance, use cover etc. but to model that you'd have to decrease the damage of a shotgun shot at more than half of its range or something like that. Effectively hitting more than one target with a single shot would be just a freak accident, imo.
As for slug rounds - they aren't really used that much aside from hunting big game, and I'd say they extend the range to about 100 meters and don't spread at all because they're single, solid projectiles. They essentially turn a shotgun into a half-assed rifle. Honestly, I've fired several shotguns with 00 buckshot rounds and the spread is simply waaaay overhyped in most games/movies. Actually, the only time it matters is when you're firing birdshot at pigeons or something like that, because 3 or 4 pellets are enough to ground the bird and you get quite a lot of them forming a neat 20x20 cm circular pattern at 60 meters or so, so it's way easier to hit.
As for knockback - there's no such thing. The target feels even less of a "knockback" than the shooter does, simple physics. Another movie thingie.