The 10rd drum has a diameter of 6.75″. Will only fit 1927-A1 Thompson models and Modern Thompson guns made in Worcester, MA.*Note will NOT fit Thompson M1 models (Military version of Thompson-if it has a side charging handle then it is an Thompson M1 Model) and Thompson guns made in West Hurley, NY or before 1990. Each drum is shipped with a drum installation device (3rd hand. Each authentic drum is entirely U.S. manufactured to the exact specifications as the originals.