If you are thinking of changing calibre...
You could get a 7.62x25 Tokarev chamber reamer and re-purpose a .303 barrel. Powerful, flat shooter which will go in many 9x19mm mags and requires no modifications to bolt or extractor. Cases can be made from .223 brass if a shortage hits (as can 9x19mm), and they'll go in .223 stripper clips of course.
The 7.62 Tokarev was 25% more powerful than its 7.63 Mauser parent cartridge, which already was the highest velocity handgun cartridge pre .357 Mag's appearance in the mid-1930s.
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