I have a 256 Gibbs Magnum cal M/S built on a 1903 Greek by Geo Gibbs.
I think the rotary mag is a replacement and a standard 6.5 M/S caliber.
It would not take and feed the 256 Gibbs Mag, which is a .470d base cartridge x 54mm.

I make my cartridges from shortened 8x57 brass. When each cartridge sits in it's slot in the rotary 6.5 magazine the extra dia makes it sit up high on the edges of the machined cut.
But dispite that, they feed and function just fine. No alterations to the rifles feed rails were necessary.
I did have to open the magazines cartridge guide ring at the front of the rotary mag a bit to allow the 160 gr bullets I wanted to use to fit and run around the rotary mechanism w/o interference. With out doing that , the bullet(s) would tip each cartridge upwards a bit as they jammed themselves underneath the guide. That brought the back end of the case upwards and caused difficult feeding and jamming.

The cartridge guide ring is removeable from the mag assembly but more easily worked on when still assembled.

I could have reduced the amt of mtr'l I removed from the guide ring by seating the bullet deeper. That would place the ogive back further and more clearance would be given. Using lighter weight bulelts would be the very simple way to do it and may very well be the answer to little or no alteration with spire points (if they feed well in the M/S!).

What this all means is that a conversion using the orig 6.5 magazine with little or no work to the magazine itself and no alteration to the actions feed rails is a very good possibility if the right cartridge is chosen.

I would be thinking of a 7mm on the orig 6.5M/S first, That would perhaps require a slight increase in dia of the neck portion of the rotary mag feed troughs.
Next would be something like a 7mm on a 54mm .470d base case.

Just my thoughts