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Can it be done? Yes, sometimes...

But to count on it, never. A 10mm headspaces on the case mouth. Dropped in the chamber, a .40 would bottom out with negative headspace. The firing pin and extractor would not contact the primer and rim, respectively. What makes a .40 work in some 10mm pistols, is upon feeding, the rim is captured by the extractor, not allowing the case to bottom out. All it takes is just one time, the extractor to not lock onto the rim, and you''d have a failure. Gee, if the pistol is operating violently as they normally do, maybe you can count on capturing the rim every time. But slow up the slide operation for any reason, and the extractor could push the rim ahead of itself and the pistol would not operate.

Have I fired .40 in a 10mm, or even a .380 in a 9mm? You betcha. But only for experimentation and I would never suggest it or even count on it to work.

And yes, if you could find an exact replacement barrel in .40, it would work. The .40 cartridge has plenty of oomph to operate any of my 10mm pistols without changing any springs.
 
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I generally consider it safe to do in a gun like a glock, that holds the round back via the extractor. I've shot a few hundred .40sw using a G29 and a G40 with 10mm mags. It worked better in the G29; probably due to the reduced slide mass in comparison to the latter, however, I would not trust it for anything more than range fun.
 
I know it can and has been done in automatics, but I would not personally do so. There is at least one 10㎜ revolver that it is safe and, presumably, effective to use both cartridges though.

But to each their own.
 
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Beyond the headspace issue and potential for inducing failures because of that, I'd imagine with regular sustained .40 usage a carbon ring would form in the chamber and make feeding 10mm problematic without a deep clean. Similar to what I've heard about .357 revolvers that only shoot .38 special.

Another note, most commercial 10mm is loaded very similarly to commercial .40 so stock, cycling shouldn't be an issue. If one were to change out the recoil springs to handle full house 10mm, I'd put money on .40 no longer being sufficiently powerful. Just buy a .40 pistol and save a potential headache and regrets
 
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I know it can and has been done in automatics, but I would not personally do so. There is at least one 10㎜ revolver that it is safe and, presumably, effective to use both cartridges though.

But to each their own.
S&W 610 Revolver is 10mm, but also shoots .40cal.
Is there another one?
 
S&W 610 Revolver is 10mm, but also shoots .40cal.
Is there another one?
That was the one I was thinking of. S&W, at one point, made the 310 Nightguard in same. I've read of, though never saw one in person, a Ruger GP100 in the cartridge. (I don't follow single-actions any more, so maybe there is one of those too. Or not.)
 
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Which is the better of the 2 SMITH 610 or Ruger GP 100?
I can vouch for the 610, 4" barrel model, as I've had it for years and shot both 10mm and .40cal out of it.
Factory and handloads.

I have not shot the GP100, so can't speak to that one.
 

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