I have absolutely no experience with the ''modernized'' versions but these may be a few areas to check. The barrel was cut down to 18.5'', right? Was it threaded straight afterward? Bullets brushing the thingy on the end of the barrel maybe.
Does the rail touch the stock anywhere? It shouldn't AFAIK.
It's my understanding that the ferrule is supposed to put slight up pressure on the stock tip when the TG is locked up. BTW, how does the TG lock up? Some force required to close it or does it close easy?