they use a 9mm thread of 1/2 x 36. Kind of a pain to find them. I’ve seen vortex in this thread. I am hoping to get a short barrel for mine and have a standard thread for 6.8 so I have more options for a hider.
Stag uses 1/2 x 36 which is actually a 9mm thread, mainly to prevent someone from using a 5.56 on it. kind of a pain though.