Hey Gino,
Yeah, we call 'em GAU-2Bs in the USAF. The center ammo bins/cans have a max capacity of 4500 rds for each gun (total 9000 rds). The only time we ever carry max ammo like that was when we were doing gun training/qual. For CSAR in A-stan or Iraq we usually carry 2000 rds per gun (4000 total). This is with the center bins/cans and GAU-2Bs (M-134s). BTW, our 7.62mm link comes in 1500 rd cans as well. we link it together when we put it in the center bins.
Now when we carry M-240s, we don't (didn't) use the center bins. In fact the main reason we retained the M-240 capability is to carry max pax (the center bins take up a lot of room). We'd strap the cans (usually smaller cans) right next to the gunners position. With the lower rate of fire usually we'd carry about 1500 rds per gun, but this would vary depending on the mission (plus whether you are high bird or low bird in the chaulk). Another benefit to M-240s is that if you went down, you still had two functioning GPMGs for self-protection. With the miniguns, no electrical power...no flame. Remember Mogadishu? Lots of ammo sitting there with nothing to run it through.
Now, with the new external gun/ammo mounts I'm not sure what the max capacity is. I'll have to get back to ya on that one. I wanna say 2000 rds per gun but I'm not sure...stay tuned.
Hope that's what you were lookin for.....plus a little extra