I am curious as to what it would take to modify typical Mauser bottom metal to feed a rimmed cartridge? I know the originals were made slanted, but how would you retrofit for this? I remember looking at an M54 win in 30/30 that had a sheetmetal piece installed at the back of the magazine, I don't remember if it was slanted slightly forward or not. Anybody done this?

If you simply insert sheet metal into the current mag box you will end up using a very short cartridge. The originals, like the Siamese, required a much different trigger in order to accommodate the rear wall of the bag box being so much closer to the rear.

Here is how the Brit's did it with the P14 converting from rimless to rimmed .303 British:

https://www.picclickimg.com/d/...FLE-MAGAZINE-BOX.jpg

Since the .30-30 case is only 50 mm in length you might get away with this strategy in the Mauser.

The rear of the box has the slanted insert and slanted guide rails at the rear for the rims and just below the shoulder forward to keep the rounds pointed forward. Basically the length of the Enfield magazine allowed a slanted mag to be built inside the trapezoidal box mag.

take 4 30/30, stack them with top round, rim in front, wrap with 2 rubber bands, and you'll be able to gather all the data you need.. then add a follower to get the angles --

should be okay for a 45/70 or 30/30 length round .. maybe ...

think lee speed or siamese mauser

Ok, actually four .303 ball rounds will easily stack into an 8mm magazine, with the rims stacked correctly. No need for a radical slanted magazine.
But that is not the issue; the challenge is the extractor, which I have not tried to make work, but Mauser rimmed extractors are different than rimless ones.

I tried 4 .30-30's in my 1916 Spanish (1893 action)and they stack well and feed up to the point of the rims sliding under the extractor. Plenty of room for longer bullets. If I had a 1893 Turk deep floorplate I might get 5 down.

In looking at the P14 extractor it appears they reduced the height of the extractor blade by about 40%-50% from the top and bottom equally. Then added some metal to the extractor nose to strengthen it.

https://cdn11.bigcommerce.com/...9.1541019730.jpg?c=2

I can visualize a raised breech like that of the Mauser safety breech. With enough clearance so only a shallow extractor notch would be needed. The generous rims of the .30-30, .30-40 Krag, and .303 Brit would provide a nifty gas seal. A nice way to use those old actions with gas venting and pressure concerns. 7.62x54 Russian won't fit through feed rails.

With handloads it would make a great truck/loaner gun. Commercial ammunition available everywhere with the .30-30.