Since there don't seem to be any drop in barrels in .357sig for the 4xxx pistols, has any one considered or had the barrel of a 5906tsw machined for .357sig ? Doe's anyone know of a shop that has done this .

Bar-Sto used to make .357 SIG barrels for 4XXX series S&Ws. Don't know if other barrel makers did. You could certainly rechamber a 9MM barrel, but I don't know that the 59XX series breech face would be big enough to allow the .40/.357case head. If you could find a .357 SIG barrel, I think you might want to start with some version of the .4006, 4003,etc, pistol due to the 4XXX series having both the correct breech face, and a bit more slide mass than the 59XX series...ymmv

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NRA Life, COTEP 640

You are so right, I completely forgot about the narrow 9mm breach face . It's a shame these guns are no longer in the catalog . I would love to have a medium frame 3rd gen chambered in .357sig .