No. a barrel is all that needs to be changed to convert the weapon to .357 Sig caliber. This is becuase a .357 Sig round is basically a necked down .40 S&W round. Actually the .357 designation for the caliber is a bit misleading. The designation was given because the .357 Sig round basically mocks up short barrel .357 magnum ballistics in a service length barrel. However, the actual diamter is .355 (9mm). Some hotter ammuntion will actually surpass the .357 magnum ballistics from a revolver with the 125 grain load.

The .357 Magnum is well known for its ability to stop human beings in the 125 grain semi-jacketed hollow point load. In fact, it is the most potent to date. The ballistics sent a 125 grain hollow point out at 1450 fett persecond with 583 ft pounds of energy. Sig was attempting to accomplish the same ballistics in a semi-automatic cartridge. They came very close with a 125 grain .355 diameter round at around 1350 feet per second. Some ammunition manufacturers actually push the round as fast if not more so. For instance, Corbon pushes its 125 grain load out of my Glock model 31 at 1484 feet persecond on avaerage.

BE WARNED, that I have seen people change out their .40 caliber barrel to .357 Sig and forget that they have converted it. In a self-defence scenario this could be very bad.