Just to clarify, the grooves are cut into the barrel, not the head.

Inner groove:

Outer dia - 65.4mm
width - 2.5mm
Depth - 1.3mm

Outer groove:

Outer dia - 103.2mm
Width - 3mm
Depth - 1.9mm

Hope the measurements are clear.....let me know if they are not.

Just tried an NF5 head on an NSR barrel and they don't fit.

I know a guy did cut his MC18 barrels to accept O-rings. It woeked as far as I remember.

Good luck