Today I removed the original John Rigby .350 EXP barrel from a magnum length M98, the one with the two-diameter receiver ring, and was surprised to learn it had small, 25mm (.984") threads. Shouldn't be a surprise, I guess, but I expected the standard large, 28mm (1.103") threads on a magnum action.

Makes sense to me given the reduces diameter section of the front ring and, I’m assuming, the very early production date of that particular action. Base and rim diameter is quite close to standard belted mag, as opposed to the much wider 416 Rigby and case etc that soon followed. Speed’s books may have something to say about.

I always just took it for granted that the Magnum actions were all large ring, Live and learn! PM sent.

Food for thought.

According to Rigby's history on their site (see 1897), Rigby bought much of Mauser's 96/98 transitional inventory as they moved out of Dublin. I think you have a very early, very rare Rigby Magnum (1900 - first year) action that may have utilized a leftover small ring barrel. Coincidentally 1900 is the first year the 400/350 express was offered. Experimental? Maybe. Tough call. No proof in writing.
CB

Yes, this is an early action. The Oberndorf Mauser serial number on the action, bolt, and small parts is 254. The British proofs on the barrel indicate it was proofed for a 300-grain bullet, not the later 225-grain bullet of the .350 No. 2 Express.
I'm quite fascinated that we, perhaps 120 years from the date of manufacture, are still interested in and concerned with the history of these older firearms. From my perspective as a gunsmith, it is indeed an honor and humbling to work on such an historical rifle and restore it to full use. Without questions, older rifles like this Rigby, have an inherent quality and feel absent in today's commercial offering, bespoke rifle excepted.
I'm now fitting the Rigby with H&H style scope rings and will report how this old girl does with a scope and modern ammunition.
Cheers,
Roger