About 4 mos ago I purchased a Smith and Wesson M&P 5.45x39 AR upper.Installed the supplied x heavy hammer spring. From the beginning, I had light primer strikes/failures to fire on military surplus ammo both Bulgarian and Russian. It fired about 80% on comercial ammo.I ordered a wolf extra power hammer spring with no change in result. Took it to a local gunsmith and he sugested the chamber was cut too deep. I called customer service and sent it back to S&W with a letter outlining the failure to fire issue. It was sent back to me with a used scratched and scribed on(with no of rnds and lines) shop mag stating the failure to feed issue was resolved new mag........Hmmmm So I called Customer service again a shipping label was sent and off it went again on nov 4.after 2+ weeks of leaving messages for the customer service rep i've been dealing with, I recieved a return call from S&W today....They now state they test fired it and there is nothing wrong with the upper I must have an issue with my lower??? Really?? It functions flawlessly with two different 5.56mm uppers,a 7.62x39 upper and 6.8 upper but not thiers. anyone else had issues with S&W service any suggestions??? I'm likely replacing the barrel unless someone here has a better idea...Anybody got anything?