Greetings @podocarp,
A friend rebarrelled one for me in the early 1980's which I still have. We used a takeout barrel from a .308 target rifle. The barrel had to be cut of at the breech around 40mm to get rid of the old chamber and rethreaded. A new chamber was cut in .30-30 which needed some adjustments to the extractor. Later the chamber was recut to .30-40 Krag. As it was a target barrel there was plenty of meat at the breech end for the new threads which may not be the case with a sporter barrel. I believe that the threads on the Martini and the Lee Enfield are similar so one of the latter barrels could be used but the whole chamber would need to be recut. This is to to provide the recess for the rim which the Martini has but the Lee Enfield does not and provide room for the case.
The only other option is a new barrel but the Martini rifles were mostly made in the century before last so the choice should be between lower pressure cartridges.
Regards Grandpamac.
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