This is the Promag Ruger American 9mm 50-round drum magazine. 

Save time and spare your thumbs with this high-quality 50-round drum magazine from ProMag!

Built with a steel upper tower and a DuPont Zytel-based polymer drum, it offers excellent durability and reliability without unnecessary weight. Inside, an injection-molded, anti-tilt polymer follower keeps all 50 rounds properly aligned and feeding smoothly through the steel feed lips.

Whether you’re breaking in a new pistol at the range or dumping mags with friends in the backyard, this drum magazine is up to the task.

Grab a drum or two and stay ready for anything!