[MileHi]

I say a take down 30-30/12 g over and under with 20” barrel. Singe shot 30-30 can be loaded stiff with 150 gr Nosler Partition and the 12 g can be slug, buck or bird shot w/ threaded chokes.

Savage (I think) used to make one without the take down and threaded choke. Someone should again.

[Yosemitest]

I've looked for something similar to that, and can remember in the 1970's, seeing several like you describe.
But those combos are hard to come by now.

[kitchen]

The is also the Valmet 412 series of rifle/shotgun double. The .222 Rem over 12 gauge was once the most prized turkey gun in Penn's Woods when there were 2 seasons (Spring and Fall) and one was rifle legal. Savage may have even marketed the Valmet at one point but I'm not sure if they were re-badged with the Savage name. They were definitely a level or two higher quality than the 24 models.