Your Browser is Outdated
Yes it can up to 9mm since threads are 1/2x28
Good question! Both Manticore and Optics Planet could be much clearer in their description. FWIW, I bought this for a 9x19mm AR build with a Faxon barrel, hoping it would be large enough for the 9mm to clear. Interestingly, the brake I received is easily large enough, and it's even stamped "9mm" on the external base at the threads. That is not at all clear from the description here.
Of course, the 9mm PARA projectile is nominally 0.355" in diameter. The brake I received is, according to my ChiCom digital calipers, 0.466" internally at the base (inside diameter at threads), and the inside diameter at the muzzle end is 0.500".
I'm quite comfortable mounting this visually impressive brake on a 9x19mm build. Hopefully, all examples are identical to the unit I received, but I can only speak to it.
If the description were clearer here, OP would surely sell many more of these. Good Luck!