Huh. Cool setup. I wouldn’t have expected the Radiant to be adding all that much back pressure, but I don’t know the Colt HRT 11.5” gas port specs. In that case I’d definitely suggest just starting with the H3 buffer, see where that gets you.
Out of curiosity, long or short configuration for the Radiant? Are you running the normal end cap or the 5.56 one?
Long with normal 7.62 end cap.