I'm not sure the Ruger flash hider is 1/2 x 28. That's the standard AR-15 thread size if I recall correctly. I tried to put an AR flash hider on my Mini-14 GB and it didn't fit. I think Ruger flash hider (at least for Mini-14s) is 5/8".
If you're talking about the SR556, then I don't know.
Hope this helps.
Rob