old cat, 1/2x28 is standard for .223. The minimum shoulder you want is .100 over the thread diameter. The thicker, the better so 9/16x24 would be preferable, but it's an odd size for muzzle accessories. I stick with 1/2 and 5/8. My .223s are 1/2 as well as my Tikka T3 Lite .243 and .270 due to the sporter contour so I wouldn't have to chop them shorter. Larger calibers/contours above .725 I have threaded 5/8.