Why, oh why, did S&W make the Sigma with a non-standard accessory rail? The answer to that question will probably be lost for eternity, now that the Sigma is past history (replaced by the SD, which has... Bingo! A standard rail!).
The adapter from Laserlyte installs easily and works as intended. An unavoidable issues is that the attached light or laser will mount about 1/2" lower than desirable. Oh, and good luck finding a holster to fit this setup.
But hey, if it's any consolation to Sigma owners, you could have spent 3 times as much on an H&K and had the same problem!
This review was written in the old system and had content requirements that are different than reviews written today.