Originally Posted by 505Gibbs

I agree with Handgunner in that its not much more than a 'hot' 9mm. Case length 2.5mm longer than the 'standard' 9x19 i.e. 9mm luger etc.

Here's more from an article by Massad Ayoob:
the .356 TSW round that Smith & Wesson and distributor Lew Horton debuted in 1995. It failed at the time because it was sold as an IPSC combo and was unnecessarily hot for that application. However, with the Cor-Bon 124 grain jacketed hollowpoint at a true 1,450 feet per second, the .356 now equaled the best .357 SIG and .357 Magnum of identical ballistics!

The .356 TSW (formerly: "Team Smith & Wesson"; now, more wisely: "Tactical Smith & Wesson") has an advantage over the .357 SIG. The SIG is a bottleneck round, a .40 S&W casing necked down to 9mm. The .356 TSW is a straight 9mm casing (9X21.5, for those who like details) and, thus, more rounds can fit into a magazine.