Ammo is electric primed too. They used to use DU ammo for the CIWS guns, not I believe it is a saboted tungsten projectile.
My direct experience with M-61s is begging empty ammo cans off the people that operated the guns, did a few moinor repairs on bit parts too.
Despite the rate of fire, I'm not impressed, since these 20mm M61 guns only have an effective range of 500 yards. (At least, that's what the plaque said on the F14 display aboard the U.S.S. Midway in San Diego a couple of years ago when I visited it.)
A most excellent question, that.
Having watched them fire in air to ground role, I am impressed. Can't shoot long, but the fire control is impressive. I would not want to be the dude on the ground.