These are high quality threaded barrel holders that come in 1/2 x 28 and 5/8 x 24 threads that are made in the US by Radix Precision.

These are used to hang barrels during the Cerakote process. They can be used for either holding the barrel when spraying or can be installed before blasting so it completely blocks the face where the muzzle device mates to. At this time they are sold as a set.