As the rifle Barnsness used had a groove diameter of .313" azs he reported, one must slug the bore to determine groove diaeter.

Note also, the .303 case has a barely greater capacity than the .308 case. Due to it's taper, it probably cannot be loaded to the same pressure in the Ruger, however that is why John raised the ante to over 2,600fps with 180's and over 2,800fps with 150's.

I dare speculate he saw nothing that could be construed as excessive or even high pressure - as long as the brass fit the chamber well, initially.