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You will have to switch slides as well, as the breech face of the 9, where the cartridge base slides upward going into battery, will be narrower on a 9mm slide than on a 40 slide.
I don't own a P320, so the forgoing is assuming the breech face is made like most every other SIG pistol.
With guns like the P229 40, one can do the caliber switch with just a conversion barrel going to 9mm, as the cartridge doesn't care if the breech faces is wider than need be. But one cannot go from 9 to 40 without getting a 40 slide.
I don't own a P320, so the forgoing is assuming the breech face is made like most every other SIG pistol.
With guns like the P229 40, one can do the caliber switch with just a conversion barrel going to 9mm, as the cartridge doesn't care if the breech faces is wider than need be. But one cannot go from 9 to 40 without getting a 40 slide.