I have both 9 & 40 Shields. I put the 9 barrel in the 40 slide and shot 3 mags just to see, without a hitch.
I am going to wait abit to see if StormLake makes an actual conversion barrel before I send for the factory 9 barrel. I like the Stainless version so one can tell them apart quickly.

I have the StormLake conversion barrel for the my 40c and works great.