A Thompson is just not a Thompson without the most famous accessory item ever – the Drum Magazine. The impressive look and feel of the Drum Magazines are unsurpassable. Each authentic drum is entirely U.S. manufactured to the exact specifications as the originals. The 10 Rd. and 50 Rd. drums have a diameter of 6 3/4″ and 100 Rd. durm has a diameter of 8 3/4″.