And the problems with those can usually be fixed of you pull the FSB, carefully realign the fsb with the gas port, (if the fsb is misaligned and pinned the the barrel, your SOL unless you use sleeve and bearing retainer loctite to resecure the FSB) and make sure the gas tube is carefully aligned with the gas key- you might have to retorque the barrel nut to get everything aligned properly.
It doesn't take a great deal of skill to index and assemble a barrel/gastube/fsb correctly, but it does take a bit of time. Some of the bargain basement uppers are put together by the unknowledgeable bubba in his garage.