Regular copper-jacked lead ball ammo is usually used for target practice. The Fifty Caliber Shooter's Association has links to sources of information about .50 BMG ammo. The M2 Browning Machine Gun is fully ambidextrous. This is important to me because I'm left eye dominant. That's why I want one.
Bullets contain ferromagnetic material concentrated toward the tail.
Jackets are copper. Bullets are not seated to the cannelure, but the case necks are crimped. The primers appear to have an old round crimp overlaid by a newer 3-dent one. Looks like the original crimp was swaged rather than reamed out. Author suspects these have been disassembled and reloaded with steel-core AP or possibly API rounds.