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One solution to getting the .30-30 to feed would be to track down original or after-market magazines for either the Remington 788 or Savage 340 in that calibre. These only hold four or so rounds.
You could then either mount these inside the case of the .303 magazine or, if you are handy with machine tools, make up a "filler" block for the magazine well, from aluminium and cut its internals to hold the .30-30 mag. I suggest starting with "Plan A".
Maybe you could find some "tired" No4 mags with damaged / cracked lips for such a project.
Another alternative is to use a .303 mag body and insert a "short" liner at the rear. The best example I could recommend is the detachable magazines for the Sako 75. The one for 6mm PPC is VERY clever.
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See also the magazine for the Stevens 325.
A "gun spare parts" company called "Wisners" purchased all of the tooling for the early Stevens / Savage magazines, and are making them again.(along with a LOT of other goodies).
See here: Savage 340 Bolt-Action | Wisners Inc
