I think, as others have said, trying a small base 308 die would be the standard procedure.
But, .488 is waaay too big.
A few observations: I'd expect an Ishapore 308 to have a mighty generous chamber, as it seems you found.
Never had a minigun myself- couldn't afford to feed it if I did.
But, I know what it is, and I'll make a guess. In something electrically powered like a minigun, rather than gas powered like an M1A, I suspect the firing/ejection cycle is happening a whole lot faster than in any gun we civilians have experience with. I'd bet the case is being yanked from a minigun chamber about the time the bullet is exiting the muzzle, and the brass is still expanding when the case clears the chamber.
Since you tried a small base 30-06 die, and that didn't help, I'd say that brass was expanded past the point where it can be sized down again. Gonna be lots of work to pull the bullets and salvage the powder on that much ammo, but, IMO, that's the only safe thing to do.