This is the Promag HK MP5 9mm 50-round drum magazine. It also is compatible with HK MP5 clones, like the Century Arms AP5.

Each Promag product is made with precision manufacturing, and this one is no different. This HK MP5 drum magazine is made of an impact-resistant polymer shell, with a tough steel skeleton underneath. Made to be durable and reliable for a lifetime, Promag’s drum magazine can weather all sorts of damage. Loading and feeding are made smooth and easy, with the addition of a load-assist dial on the front of this drum magazine.