A fast-attaching, self-aligning, easily adaptable firearm adapter. The novel adapter attaches to the barrel of a firearm while precisely aligning the barrel and adapter using a concentric constriction sleeve along the barrel and a barrel guide to funnel firearm muzzle. The device is adapted to attach to a wide range of barrel diameters via an easy to fit and inexpensive to manufacture fitment sleeve. Thus, the adapter can be manufactured in a single size and work with most firearms. In addition, the device attaches to the smooth section of barrel thereby eliminating the need for permanently modifying a firearm by threading the barrel.