i am far from an AR specialist. most of my dealing with the m-16/m4 system was toting it in very hot or very cold parts of the world and dealing with the friendly natives.
the AR is one of the most simple and easy to assemble and smith rifles ever designed.
two things it has to have -
# 1 a good barrel assembly that has the proper chamber and head-space.
#2 a properly made bolt and bolt carrier
these are not places to skimp even on the most low $ effort, AR-15, AK, FN-FAL don't matter the type--they all have to be correct in this regard.
this is not saying that there arn't some good venders in the lower priced AR market. i can list some good ones.
be very wary of vendors that are raiding others scrap piles. it is that last dollar that many try to save that gets them into this kind of shit.