That’s interesting. Admittedly, I don’t own a .40 auto to examine extractor fit, case head fit to the breech face, etc. Even though I have hundreds of rounds of .40 S&W in my collection, and thousands of 9 mm, I have not measured the case head dimensions. I will do that when I find time. It is hard to believe in the interchangeability of the two calibers to that extent, but then, I haven’t tried it. Perhaps my own belief that since the cost of the barrel, etc. can buy a lot of ammo for the original gun (or used to anyway - who knows these days with all the hoarders driving up the price of ammo). I never found the attraction to such conversions, especially if they are on the “Mickey Mouse” side of the equation.
The Colt Pocket Hammerless was a no-brainer. The two cartridges, due to the very pronounced semi-rim of the .32 auto cartridge. are very close to each other at the head (that is, the .32 is close to the .380). Many companies have used the same magazine for both calibers - CZ 24 and CZ 27, FN-Browning Models 1910 and 1922, etc. The Czechs marked their magazines different - I don’t recall seeing a caliber marking on the original clips for the vz. 24, but the mags for the vz. 27 were marked with the model, as I recall, but I have used the Mod 27 clip in a Model 24 myself, with good success. I owned a Model 1922 Browning clip that had “7.65” on one side and “9 mm” on the other, both factory-stamped. Slovenia converted a few Makarovs to .32 Auto (7.65 Browning) with nothing more than a new barrel. They even used the 9 mm clip for the 7.65 mm cartridges, although with the advice that you must load only 7 rounds instead of 8. I have tested that use, although only by loading a magazine, since I have no 7.65 mm barrel for a Makarov to try the feeding, but was told by a Slovenian collector that his specimen of pistol worked just fine.
So, I guess as usual, I engaged my mouth before I did my brain; that is, I went with gut feeling and did not research my answer. What is it they say about “a little knowledge?” Perhaps it is a dangerous thing.
I will satisfy my curiosity as my son has his CHP issue .40 Auto, and I will have him bring it down to the house one of these days just to check extractor fit and the like for myself, although I don’t doubt Dan’s words. He has forgotten more about the mechanics of firearms than I will ever know.