Abstract: Adapters for mounting an accessory on a firearm, firearm accessories configured to mount to those adapters, and sheath or handguard assemblies of firearms are disclosed. The recited accessory adapter is substantially annular or tubular in shape, and includes at least two portions that are arranged adjacently along the barrel axis of the firearm. A rear portion is formed on the weapon side of the adapter, so that the rear portion is geometrically complementary to the front end of a firearm sheath, at least in part, and a front portion of the adapter includes a cylindrical coupling surface on the outside for coupling of the accessory, where the coupling surface includes at least one bayonet extension that projects radially outwards.