Every product that is manufactured will have a certain percentage of failures that slip past QA. What distinguishes brand x from brand y is the amount. In the case of Norinco's, how many threads are posted demonstrating failures or problems. Quite a few. How many threads do you here of SA rifles having a failure? Not very many. Unfortunitly you won the lottery - in a bad way. I believe SA has a good warrenty of their rifles and hopefully they will still be able to help remedy the defect. I think all in all it's safe to say the SA makes a better quality rifle than the Norinco but sometimes failures aren't detected by QA and a defective rifle ship regardless.
Furthermore I would also like to add that even the most premium M14 receivers are subject to flaws. For those of you that think LRB is the cats ass, do some research and there are threads showing LRB receivers that are not within USGI spec. Again, not saying LRB is crap, but everybody ships a defective product that makes it through QA. It's only a question of how much. In the end you get what you pay for.
Just tossing this out there but some people are reluctant to admit to failures in their masterpiece of American Engineering. I mean......its American right, its gotta be better than that Chinese crap right and it cost 5 times more so its gotta be better ...........right ?