Think of it this way, theres no point to having a full metal gun when it doesnt work for w/e reason.
in one of my freinds G&P M4 he had the following problems:
-hop up was screwed and needed to be replaced (out of the box)
-since it was shooting 410fps the spring was fucking up the gears and wouldnt fire right
-mags didnt feed properly
He had a couple other probs but i cant remmber right now.