A firearm barrel fitment sleeve and method of use. The fitment sleeve is preferably fast-attaching, self-aligning, and securable without the need of a tool. The novel fitment sleeve allows a single firearm accessory adapter to be used with a wide range of firearm barrels by temporarily modifying the size of the barrel diameter without permanently modifying the structure of the firearm. The fitment sleeve is inexpensive and preferably includes a longitudinally extending slot in the lateral wall making a single fitment sleeve usable on a wide variety of barrel diameters. Additionally, the fitment sleeve is heat resistant to avoid melting and/or catching fire when the barrel increases in heat during operation.