Lone Star Armory lightweight 1/2-28 thread protectors.
Machined from billet aluminum and hardcoat anodized. For grip, the outside is either knurled (and radiused) or has machined flats (square on the back, chamfered on the front).
Available in three lengths:
5/8 (.6285) Knurled
1/2 (.5020) Knurled
7/16 (.4365) Knurled or Flats
The 5/8 is long enough for AR barrels. The 1/2 sits just a little past SilencerCo and Lone Wolf Glock barrels, The 7/16 fits KKM threaded barrels perfectly, and will fit nicely on most pistol barrels with an O-ring.