[Sevens]

I am not doing this -- 10mm cartridges headspace on the case mouth so shooting .40 S&W brass through semi-auto pistols chambered for 10mm is asking your extractor to do the job of holding the loaded round to the breech face. It may or may not work, but it's certainly going to stress the extractor and ask it to do something it's not designed to do.

Next up for me would be the case mouth tension of .40 S&W brass that is designed to have a bullet seated to a certain point -- I believe it would be extremely unlikely that you'd get proper mouth tension on that bullet seated out so far and that it would most likely want to or try to seat itself further in the case. If/when that happened (typically as a fresh round being violently chucked in to the chamber) you'd have a nuclear bomb as a 10mm load and a 10mm slug would now be contained in a .40 S&W piece of brass. The pressure would shoot up exponentially and you'd be begging for a catastrophic failure.

Not to mention any damage you might do to the mouth of the chamber if everything else went off without a hitch.

Yeah, .40 S&W brass is everywhere and cheap as dirt but if you want to shoot 10mm, you've got to pay to play.

*NOTE* Obviously, I'm completed against the idea but will admit clearly that I've not ever tried it in either of my 10mm pistols. My advice is nothing more than opinion with no hard evidence to back it up.