This is why the decocker works. There is a physical block preventing the hammer from contacting the firing pin when you decock the weapon with the decocker. This block is removed when the trigger is pulled, because the designers assumed you were going to fire the weapon. Thus, if you "thumb down" the hammer, the block is removed. If your thumb should slip . . . KERBLAM!Advantage with the Sig's (M11) is that it's got a floating firing pin. It won't (shouldn't) discharge if you whack the hammer with a round in the chamber.
Oh, and if you want that .45 so bad, maybe Uncle should order these . . . :icon_tong