Centurion Arms LLC, C4 5.56 Forged Receiver Set 7075-T6 Aircraft Grade AluminumReceivers Machined from Aluminum Forgings 7075-T6Receivers Hardcoat Anodized per MIL-A-8625F, Type III, Class 2M16/Auto PocketThermal Fit inner dimension measures at .997 -0.0/+0.001" and requires the application of heat in order to provide a solid mate to the recommended barrel extension. By heat we mean more than granny's hair dryer.