I've given up on the idea of a S&W Grail gun because the amazing variety in old modified S&Ws is a font of endless delight and surprise to me.
However, if I were to have a grail gun it would be this:
It may be a fantasy piece built by someone in the 1960s, or actually a German version of the DeLisle carbine. I don't know, there's scant information about it online, and what is there is supposedly second hand from some mythical gun magazine in the 70's filled with stories of Hitler assassination plots and Goering's personal gunsmith.
Whatever the case may be it's the gun that comes to mind when someone says Grail Gun to me now. I fully believe that I will be able to own all the fine S&Ws I want, but this gun? This is a grail that will never happen, maybe because it doesn't exist, maybe because it does and it has no paperwork, or maybe just because if it did come up for sale it would sell for so much money I couldn't hope to buy it.
In any case a takedown, grip safety, bullpup, turret scoped, integrally suppressed, 9mm, Luger magazine using, double set trigger rifle is just way way too neat not to desire.