Semi-auto pistol cartridges head space on the case mouth. When a 40 S&W round is chambered in a 10mm, it is the extractor that holds the head space. This leads to variable head spacing and combined with the extra free bore you can get poor accuracy. Also, extractors are designed to extract the cartridge, they are not designed to prevent the cartridge from dropping into a long chamber during firing. Firing 40 S&W in a 10mm places additional stress on the extractor hook. I'd reserve shooting 40 S&W in a 10mm barrel for emergencies, I would not make it a practice, but that's just me and my desire to not break a S&W extractor.

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VCDL, GOA, NRA