This question might generate some flack, but I'd like to know. Since this thread is a little old I'll give some background for my question.
The original question was essentially, can a 40sw cartridge be fired from a 10mm pistol.
Robby1911 said "...the 1911 cartridge headspaces off the extractor. you can shoot .40 without changing anything. the reason for a new barrel is to avoid chamber erosion..."
Shoot1Sam said "...1911's do NOT headspace off of the extractor; as glocks rock stated, they headspace on the mouth of the case and if only extractor is holding the cartridge, bad things can happen."
sqlbullet said "Shoot1Sam is exactly right. And if you want an idea of what can happen when you shoot .40 S&W in a 10mm chamber, look
here.
The sole exception to this is the Springfield Omega/Peter Stahl 1911's which featured dual extractors. Because of the dual extractors, these guns effectively did headspace off the rim. The impact of the firing pin would not push the cartridge deep into the chamber before detonating the primer pellet."
The answer to my question may very well be the last line of sqlbullet's comment, but I'd like clarification. So the question is, why is a single extractor holding the shell not enough to allow it to effectively headspace off the rim?
Thanks...