............ if you fired a .40 S&W round in an auto chambered for 10mm??

I'm half embarassed that I don't know. It's going to go bang. But what does the excessive headspace on the casemouth do or cause??

Maybe you ought to email Guns and Ammo TV and see if they will do it on the torture test segment!

Lower velocity and a very dirty chamber. Other than that, not much.

Accuracy is a bit less, but the big thing is reliability- the headspace is being maintained by the extractor, which works most of the time with most guns.

The combination of dirty chambers and extractor headspace made a really reliable 10mm into a so-so 40 S&W.

I tried this in IPSC because a bunch of low- lifes would pick up my 10 mm brass and I could hear them say that the brass was worth a lot, and they had a friend or relative with a 10mm so lets give this to them....

pretty much what charles says, rmember the s&w 610? it was a revolver chamber for 10mm with moon clips to shoot 40's in.. at least with mine the 40's leave a dirty chamber that has to be cleaned before loading 10mm's

Thanks for the insight fellers.

I guess my main reason for asking is that I have both a 10mm and a pair of .40's. Always wondered if the chips were down and I had to run some .40's through the 10 if it would be a workable plan, short term.

have you considered fitting spare barrels in them to allow shooting both?
Once upon a time I owned a Colt 10/40. Same two barrel set up.

Rich