My first thought is to modify a .223 case, cut it at the shoulder and use that as a basis for a custom cartridge. This would be a rimless .357 super max! A 223 magazine would(should) work perfectly.
Downside, headspace off of cartridge mouth, so keeping cases trimmed to tight tolerances.
edit: I suppose if you were gravitating towards a real thumper you could always start with a belted magnum as the parent case.
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