Have a pc 40 that I put a Kkm 9 conversion barrel in and my extractor is far too loose to hold a 9mm case. What haw others done to fix this? As far as I know they only make one extractor for all calibers.

I do not have this problem with my kkm 40 to 9mm threaded barrel and the PC 40l.  I did have the extractor crack after one of my pistol defense classes.  It definitely had a flaw in the original extractor. I opted to changed the broken one out with an apex extractor and it is definitely tighter than the original extractor.   I went through a carbine class recently and used my 40l as my transition weapon. It didn't hiccup with a bunch of rounds during that class and test firing at the range after several hundred rounds.

I run a factory 9mm barrel in a full size M&P 40.  I get double feeds, even using factory 9mm mags.  That's my only experience.

Thanks for the replies.