1/2x28 is the standard thread size for a .223 AR15 and perhaps the error was just that, an error. Tim if you still have a threaded H&R barrel would you be able to check it for us, as well as anyone else that has that ability? If those members could post thier results here it would help us get the correct info. It would have been very easy to turn down and rethread a 9/16x32 muzzle to the more common 1/2x28, of course in a .30 caliber that would leave less "meat" at the end of the barrel. However I am sure 1/2x28 taps and dies are more common that 9/16x32....<><....
Thanks to any and all for thier help in this....