This is an adapter made by PQP to use on a threaded barrel, so that a muzzle tuner can be threaded to the end of a barrel without having to machine a block for the tuner to clamp to, and allows for the tuner to be purchased un bored (.750)

Thread pattern/size is 1/2-28 which should be the same as most threaded factory barrels.

Length is 1.25"

Diameter is .745