5/8"x24 I got mine from Jeff at Grizzy Gun Works, stainless and black.
Thanks Skypilot. Do you recall what "Diameter of Muzzle Cap" and "Length of Muzzle Cap" you used when you ordered from Grizzly Gun Works? How does it fit on your fcp-k (223?) and if you could to it again would you change these measurements?
Thanks again Skypilot,
Cal.