Firing .40 S&W in a 10mm chamber is a very, very, bad idea. Don't do it.
A rimless pistol cartridge head spaces off the case mouth in the chamber and is supported at the rear by the breech face. The shorter .40 S$W will do one of two things in the longer 10mm chamber and both are bad news.
- The cartridge will head space off the case mouth, creating a unsupported space between the case head and the breech face. The space is called "standoff". If you're lucky you'll blow a primer, if not, you'll blow the case head.
- The cartridge will head space off the extractor. The firing pin will drive the cartridge forward creating standoff and once again, a blown primer or blown case head is in your future.
In either case, if you're really unlucky, the fired case will be propelled backwards against the breech face and re-seat the extractor. The case will be ejected somewhat normally and fool you into thinking you're not living dangerously.
