The true calibre, ah now THAT is the $64,000 question!

The "bore" may have in fact been planned to be .242", while the groove diameter may have been hostage to various barrel makers machine tolerances in the rifling process?
As I mentioned, my old original ~1930's Kynoch ammunition has bullets of very long taper ogive and a base diameter of .2475"

Various sources claim barrel groove diameter of .249" to .253" while my rifle has a groove diameter of .255"
I don't have proper small diameter plug gauges to actually figure out my rifles true "bore diameter" at the top of the lands.

SoooooЕЕ.I'd opine that calling it a .242 is as good as anything.
I would be curious to know how those original .2475" bullets shot out of my rifle with the .255" grooves with rifling that "may" be only .004" deep?
100 yard "minute of Blackbuck" accuracy? Bullets with thin jackets actually soft enough to slug up a bit in the barrel?

- Mike