First of all: I know the RoF is limited by the engine (afaik to 1 round per frame).
Still, RoF for certain Autocannons/Gatlings is wrong and too low and for others it seems the possibilities of the engine are not used fully. The 6.5mm/7.62mm Gatlings have a pretty decent RoF and have the workaround with the area damage. That's pretty much ok, still the RoF could be increased to the maximum possible of what I guess should be 30 RPM.
I say 30 RPS (rounds per second) should be the least possible, because that's about the minimum FPS I like to play with. So I configurate the game and play on servers that provide at least 30 FPS. And that's no problem with the improvements BI provided lately!

Highest RoF in the game have the Wipeout Gatling and the small caliber Gatlings. These have a RoF of about 70 RPS. So these should have the highest possible RoF in Arma 3. All the other Autocannons/Gatlings then should have their real life RoF - calcuated down so it fits to the Arma 3.
(Like setting 70 RPS in RL and 30 RPS in Arma 3 to 100% and then calcuate down).

More guns I figured out firing at a much too low RoF:

Buzzard 20mm Twin Cannon:
2600 rounds per minute in real life

Blackfoot 20mm Gatling Gun:
1500 rounds per minute in real life

(Most likely the Gatlings of the Kajman and the new To-199 too, but I have not figured out yet what kind of cannon they have in real life. Maybe someone can help with that.)

The low RoF of these guns also have an impact on gameplay and realism. It causes them to be MUCH less powerful than they are in real life, making e.g. the 20mm Twin Cannon of the Buzzard pretty much useless!