Assuming you just want to cut the existing barrel and refrain from relocating the locking lug and shortening the ejector rod and center pin, I don't see how you get to less than three inches -- which is actually a pretty good snubby length for an N-frame revolver.

It's worth noting that in recent decades S&W have produced N-frame models with 2-5/8 inch barrels and even two-inch barrels. If the barrel threads haven't changed in the last century, you might even get a modern .45 barrel and just screw it in, though you would still have to do something about the ejector assembly. That sounds to me like more trouble than it is worth, but then I am older and less adventurous than I was 8-10 years ago, when I might have been tempted to try something like that.

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David Wilson