My buddy owns a 9mm Shield and I own a 40 cal. Shield. We were cleaning our guns and got the idea of swapping out the barrel of my 40 with his 9. Guess what it fix perfectly, exact same fit as the 40. Loaded up a 9 mm magazine into the 40 and successfully manually loaded and ejected every round flawlessly. We weren't at the range so I can't tell you how it shoots but I can see no reason what so ever why it would not.
My question is has any one tried this and what were the results?

Welcome to the Forum!
Members here have been posting about it and trying it pretty much since the Shield came out two Years ago. The 9mm barrel does fit into the Shield40, but the .40 barrel needs a slight modification to fit into the Shield9.
There are several Youtube videos online and threads here (should you attempt a search).

The only problem with finding a 9mm (conversion) barrel for your Shield40 is that so far, nobody is selling 9mm Shield barrels... No even S&W.