When I measured it myself, my figures were a little different from the gunsmith's as-converted-from-metric-into-English. (I can't find them right now, nor my calipers. <sigh>) The closest match to mine was, if I recall rightly, the .350 Westley Richards, a really esoteric and short-lived British round, which theory obviously made no sense.

I was so confused by all that that I have just left the rifle sit here, until I find a better gunsmith.

I guess I have recovered from my fit of bemusement, though, since I just bought another rifle of unknown chambering.

Sorry to put you through so much trouble.