40 to 9 conversion

[Johnmuratore]

The compact has a 3.5" barrel, so you'd have 3/4" of barrel sticking out of the slide but my guess is that it would work.
Ugly is in the eye of the beholder I guess.

 

[Jiman]

I'd imagine it'd be a little 'rattlely". The last 1/4" or so of the barrel flairs as part of forward barrel lock up.

Putting a longer barrel (I.e. One not designed for that model) is going to negate forward barrel support.