1.185" - is a very tight standard indeed, Rod. That's what 3cm is. My own personal standards for a hunting rifle are 1 1/2" rested (3.8cm) with whatever sigths it happens to have. That includes the .312 Express and my .375 Lever gun, Winchester M94.

Relevent to this discussion, my .312 Express, although having a .315" (or .314"?) groove diameter, shoots .312" Hornady 174gr. bullet into 1.5" with the factory open sights at 100 meters - #4 issue. I'm driving them at 2,960fps(re-chambered).