These fixtures are designed to aid in pressing out stubborn thermal fit barrels.
They come in two versions. One for AR15s with standard, milspec threads, the other for AR10/LR308s
AR15 - 1 1/4"-18 threads *
AR10/LR308 - 1 7/16"-16 threads **
* AR15 version will not work with certain HK upper receivers
**AR10/LR308 version will not with with certain Armalite pattern receivers with 18 TPI threads