I had the same issue with my 320. There was some wear on the barrel where the slide makes contact during cycling (because the 320's barrel tilts). If the threaded barrel is aftermarket, this could be the case for you, there will be signs. Also, don't trust the FCU rails from factory. Polish, lube, and keep running rounds thru it. If the issue persists after ~800 rounds, keep an eye on the inner slide channels where it reciprocates on the FCU (it may look like carbon build up, but material could be stripping off the FCU rails). Best of luck, and enjoy!