Ok here's the deal I have an EAA Witness 40 cal. I put an extended and threaded 9X21 barrel in it to use with an AAC silencer. Seems to work excellent with the 9mm mags I have from my Silver Team.  Obviously the bolt face on the slide is 40cal. The extractor seems to hook the shells ok and I can cycle and eject them manually.

Anyone see a downside to this?  I am shooting 161gr 9mm bullets subsonic. I have shot a couple mags through this with no sign of problems.

Another option would be to re-chamber the barrel to 357Sig then back to 40cal mags and the bolt face problem would not be an issue. Would have to customer load the ammo with the 161s but that's just fine.

Is the 357Sig chamber deep enough to remove all the original 9X21 chamber?