It says they're standard NATO rifle rounds, which would mean lead, or at best, FMJ lead bullets. Most likely FMJ since I'd hate to think how fast you'd foul the barrels when blasting 3000 rounds per minute.

If you want a better idea of what Dillon's minigun can do, check out this video:

Jesse James wastes an RX7 convertible on Dillon's test grounds with a Suburban mounted minigun

Oh, one thing to keep in mind: Only 1 out of 5 rounds is a tracer. For every 1 you see, there's 4 more you don't.