I
just - last week - put a 9C slide and barrel on top of a 40C frame, and the 40C slide and barrel on top of a 9C frame....
(Of course, I had all the necessary magazines.)
Worked fine.... Dimensions ought to be about exactly the same - or at least within tolerances. It appears that the guns are identical except for the breech face and the bore size.
The reason? The 9C came through with a thumb safety, and I'm a 1911 guy, so I decided that I wanted that on the 40C I carry around the house. I was a little concerned - the sear block on the 9C has a headed sear pin, and the ejector is a little different, but everything worked. I was prepared to swap sear blocks and figure out how to install the thumb safety in the one from the 40C, but it wasn't necessary.
(FWIW, I, too, find the thumb safety kind of silly on this gun, but I'm used to it, and I didn't want it on the 9C. That one's supposed to go to my daughter if she doesn't stay in New York.... My problem is that I think it'll turn itself ON on it's own, and if you're not used to it - there's really no need to leave it on - you've got a problem. Since I'm used to a thumb safety, no problem.)
One funny result. The 9C tended to toss empties up over my head, while the 40C sent them off to the right. Now they both seem to toss 'em over my head
....
Someday....
It appears that the .357Sig barrel is all you need for that conversion, btw. The magazines are compatible. Haven't tried it. (Can't think of a good reason, but that's another thread.)
The .45 versions are NOT the same sizes, AFAIK. I don't think you can put a Compact slide on a Full Size frame, either, although....
Regards,