Triad is correct ...
The bigger Vulcans could be electricaly, hydraulicaly, or pneumaticaly driven. They could even be gas driven by bleeding off chamber pressure to drive the barrels, (but the gun needed to be initiated by an external drive source). Also, different rotating cannons could use either electric or percussion primers. It depends on the application. As I understand, there is work being done to build a single gatling gun design that will fire either percussion or electricaly primed ammo. (Navy prefers percussion ammo, Airforce likes electric ammo). The reason for electric ammo is the fact that the design requires fewer moving parts, and is therefore a more reliable gun (assuming, of course, that your firing voltage is always available). Also, on an electric gun, there is nothing to 'cock', so there's not a need for hammers or strikers and such.
Cyclic rate would be an easy thing to vary regardless of the drive system (the M61A1 has two rates of fire). The problem is, as already mentioned, the weight and the requirement of carrying a vast amount of ammunition. This was the reason the Gatling design was ditched shortly after the turn of the last century. The Maxim gun was transportable and, although much slower than the contemporary Gatlings, it threw enough lead to meet the need ...