I have an FA mp5k clone which used to run well and now stove pipes a lot. I have tried different ammo and bullet weights to no avail. I have tried replacing the extractor spring which did not help either. I have tried replacing the entire bolt head and the ejector as well, you guessed it, still having problems. It stovepipes 2-3 times in each magazine. yes im using hk mags. any ideas?