Originally Posted by samurairabbi
There are headspace tolerance considerations. There is far more play in the gap of the extractor holding the rim than there is in the chamber holding the cartridge MOUTH. That is equivalent to much greater headspace variation, even if the .40 round DOES fire consistently. Field strip a Glock, place a case in the extractor, and wiggle it fore and aft. THAT amount of wiggle is now the headspace variation. Would you tolerate that much headspace variation when shooting 10mm in a 10mm factory barrel?
"Getting along" with shooting .40 in a 10mm barrel is something I did (before scrounging conversion barrels), but I never contend it is an equivalent condition to shooting 10mm cartridges.
Of course, but the clearance is insignificant. The minimum/maximum case length tolerance is a LOT more than the mere couple thousandths of an inch behind the extractor (case rim engaged). Chamber depths can vary, between .992-.998" (SAAMI is .992"). Minimum case length is about .982" (most new brass is in the .985" neighborhood). The extractor will hold tension on the case, defaulting to breech contact in almost every case. In any case, it's a moot point, unless something is broken. So, yes, I do tolerate this head space variance. So do you, if you shoot this pistol. For the same reasons, it is recommended (by some) that the extractor be modified to accommodate this when using a G21 slide to fire 10mm or .40 rounds (in an appropriate barrel, of course). Unless something is broken, it will be well within the range of tolerance.
In my RBH revolver, the chamber depth is .992". After a few firings of the same brass (moderate to max-pressure loads), the brass will be stretched to .992, due to this condition. Light loads have no affect. It has no extractor to hold it back. When fired, the firing pin pushes the cartridge forward to case mouth engagement, then it slams back to the breech. This is normal for this gun, by design (or lack thereof). Firing 10mm or .40 in my 10mm G20 does not do this, at all. No stretch. Many other revolvers do not experience this, as they are head-spaced using moon clips, not relying on case mouth engagement (which is a better, but less convenient strategy).