This 100rd drum boasts a diameter of 8.75\". It is specifically designed to fit 1927-A1 Thompson models and Modern Thompson guns that are produced in Worcester, MA. Please note that this drum does not accommodate Thompson M1 models, designated as the military version. If your gun features a side charging handle, then it is a Thompson M1 Model. Moreover, Thompson guns manufactured in West Hurley, NY or prior to 1990 are not compatible with this drum. Each drum is accompanied by a specially designed drum installation device, also referred to as a 3rd hand. All of our authentic drums are entirely U.S. manufactured, adhering to the exact specifications as the original products.