This is the Promag AR-15 7.62×39 50-round drum magazine.

Each Promag product is made with precision manufacturing, and this one is no different. This 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.