Here's the deal.
Using 1/2" threads on bores larger than .223 is a bad idea due to reduced wall thickness. That's why we do not recommend it or make specific ".30 cal" muzzle devices with 1/2 threads. Yes, we know some rifles come that way and that doesn't change our assessment that it's not a good idea. If you drop such a rifle on its muzzle while a can is attached there's a decent chance it might fold right there where the wall thickness is least. Having that last part of the barrel so thin is just not a great idea.
That said, although we can recommend strongly against doing it and not warranty it or guarantee it, we can't prevent people from doing it.
The actual aperture size on the 1/2-28 SR brakes will be larger than .223 because the Dominus-SR system is designed to be used "as is" on both 5.56 and .30 cal F/A or SBR's; however, the 1/2-28 parts will not be labeled for use on .30 cal bores.