O know you have doubts about the 30/30 cartridge but I always thought a conversion would go straight across. You could always cast a bullet to match and the cases would be easy. The pressures are down and a tight chamber would maybe help brass longevity. I had always thought about turning a .30 cal barrel to proper contour and then fit full wood for the original look. The mag lips would need alteration.

Short of that how about 357 magnum? The extractors are more forgiving that you think. I used 9mm in a LE action and it extracted without any alteration. 7.62x39 would be another excellent choice I think.
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