Mercenaries, Spies, and Private Eyes rolls 1d6 for every six bullets.
GURPS used to have a chart that compared margin of success for each group of 4 bullets.
Twilight 2000 second edition rolled for each bullet but applied difficulty modifiers to the number of bullets instead of the target number.
Another variant is one extra roll to hit per n bullets which is basically how Mutant Chronicles did it.
HERO and GURPS 4th edition use margin of success divided by a number. In HERO it's 2. In GURPS you get a bonus to hit based on the number of bullets and a divisor based on recoil.
For Galaxies In Shadow, I've played with a lot of ideas but finally settled on dividing the chance to hit by the number of bullets and dividing the roll to hit by the result. Admittedly that's a game where I said "screw it, just do the math" as a core principle so it'll never be popular.
For most games, offering the option of a bonus to hit one target (small arc spray), a damage multiplier after armour against one target (focused spray), and multiple targets (wide arc spray) is probably adequate. Palladium and Rolemaster Standard System both do it that way.