The Sigma is Smith & Wesson's venture into using synthetic materials in gun construction, using high-strength polymer material for the frame.
Created in 1994, it also incorporates a self-cocking or double-action only firing mechanism, so that the pistol can be fired without delay or preparation. The basic model is chambered for the .40 S&W cartridge, but it is also available in 9 x 19mm Parabellum and a sub-compact model in .380 ACP was also manufactured. A limited amount of these guns were also chambered for the .357 Sig cartridge.
The Sigma series pistols are so similar to the competing Glock pistols that Glock sued Smith & Wesson for patent infringement. The case was settled out of court in 1997, with S&W agreeing to make alterations to the Sigma design and pay an undisclosed amount to Glock.[1]
In 1999 S&W improved the Sigma series with the main changes being shortening of the barrel and the slide by 0.5 inches (12.7 mm). Other improvements included a more ergonomic and comfortable grip and a slightly enlarged ejection port, due to the proprietary accessory rails located at the front (an inexpensive adapter can be attached to turn it into a standard picatinny mount).[2]