[Hectic]

Most of those china guns will need part right away or at least prety soon after they are used, however some last a long ass time. Most cases its a crappy piston and or piston head, inner barrel and hop rubber. Gears are often prety soft and the tolerences useually suck too (ie some parts wont like to fit, even full drop in boxes may need some work to fit.)
If a 416 is what you like (my favorite AR thle rifle) then id suggest saving up and getting a vfc/umarex. They are the nicest 416's on the market, they need a piston and a sorbo pad but otherwise they are solid.