I know that you can convert a .40 to 9mm with the proper drop in components. Can you convert a 9mm to .40?

Usually, no. With many, the chamber section of the 9mm is slightly narrower than the chamber of a 40.
So, Going from a 40 down to a 9mm with factory barrels, there will be a slight amount of 'slop' (but not enough to get anyone's panties in a wad), while going with aftermarket barrels, the fit would be optimal. The 40 Chamber though, would not fit into a 9mm slide without first doing some grinding. It's too wide for the 9mm Slide.

The other factor seems to be the extractor fit. The 9mm base diameter is a bit smaller than the forty, but again, not so much so that the 40 extractor can't get a decent bight on the 9mm case. If you could get a .40 barrel into a 9mm slide, the 9mm extractor would have a tough time getting a good bight on the .40's case.

I hope I explained it properly. My knowledge comes from reading the dozens of 'Conversion' threads on the forum, as opposed to actually trying it. If I didn't explain it correctly, I'm certain someone will correct me.

Rob ,This works for me, just didn't find enough information on the web and I have seen bunches of information going from .40 to 9 just not the other way. Thanks for the knowledge, I can always use the information

have a 40c in the mail which should be here Wednesday and already have the Storm Lake 40 to 9 conversion barrel and a couple of 9mm magazine's .. my next buy will be the 357 sig drop in barrel for the Compact .. you can use the 40 magazines with the 357 sig cartridge no need to buy any for it ..

Will buy a different pistol case to keep the 9 barrel and mags in also the 357 sig barrel ..

can't wait for it to arrive so I can take it to the range ..