This is a surprising discovery; this rifle's groove-groove measures 4 grooves and .255", a sample original Kynoch bullet is .247" diameter while the commonly stated bullet diameter was supposed to be .249".
Makes me wonder if the .242 Vickers became unpopular because of bullets essentially rattling down the bore and thus not being too accurate?

The up side of this is that a common .257" bullet is easily sized down .002" in a Corbin reduction die. Case necks may need inside reaming or neck turning........we shall see.
-Mike