Re: Smallest Factory threaded Pistol?
If you think that Sig is small you have choices. But that Sig imo is huge. But if he was looking for a truly small platform, it is not easy. For factory barrels you get down to a Walther in 9mm or Kurz. The last truly small .45 was the Semmerling XLM and are simply none to be found now.
For most, the barrel wall is too thin to support threading for a can. You will hear about folks welding on the muzzle of some of these barrels to support threads...don't do it. Aside from high breakage will real use, the slide has to modified as well. Best to have a true custom barrel made (slide is worth moding then.)
Still, the small Glock is a good way to go!