"I must say this gun out of the box, is an exceptionally well built rifle. bought it a few months ago. The only complaints I have about this gun is I can't figure out how to get to the gearbox to upgrade it, and Cyma putting the excessive and unnecessary amount of glue on the flash hider, but that's just Cyma. Externals are amazing, being made out of steel, this is definitely not going to go down anytime soon. I currently have a tritium night sight, tracer unit, and a strobe, as I like playing Night time ops. Sling mounts aren't going to break, but do be weary of the rear sling mount as it's a cowbell if shaking around too much.
"This is my follow up gearbox review on the gun. After figuring out how to remove the gearbox and go into it, I can give better internal internal info on the gun. Starting with this... The gearbox shell seems to be good quality, some kind of standard black painted metal that mostly all gearboxes are. The gears are standard but strong, I'm pretty sure that they're 18:1 ratio or something standard like that. The piston is a standard CYMA one, being made out of a durable polycarbonate material, it only has one steel tooth but it's still strong nonetheless. The piston head is also polycarbonate, is decently ported, but mine had a rather small o-ring (which results in not so great compression between the piston and cylinder). You can stretch it out over something to make it bigger, or just go with a #14 o-ring which can usually be found at a hardware store. The silver cylinder is excellent quality, being made out of steel I think, and it is not ported. The cylinder head is a standard polycarbonate CYMA one, has ok compression, and has one o-ring. The air nozzle is the only kind of sub par component in the gearbox as it's some kind of clear polycarb material and doesn't have an inner o-ring, but an upgrade one can be bought for like $8 or something. The spring guide is a strong metal and has 3 bearings on it, which makes it rather hard to push the spring back in. The spring is a stiff M120 which can also be hard to push back into the piston when reassembling. The anti reversal latch, tappet, tappet spring, trigger set, and trigger springs are all good quality, and don't need any upgrades. Oh, and the trigger contacts are ok, I haven't really messed with those much and I think they'll be fine.
Overall, all I would really recommend upgrading would be the air nozzle for better compression and durability, and maybe a weaker spring if you want to have an easier time reassembling the gearbox. I bought an M110 Matrix spring, and I must say that it's even stiffer, longer and harder to put back in than my stock M120, you may want to go with like an M100 of a different brand for a spring downgrade. You may also want to re shim the gears, which isn't really all that hard, and re grease everything with Teflon grease to make everything run smoothly. So that's my run down of the internals, they're overall very good for the relatively low price of the gun, and they need little to no upgrades. They should do you well and perform pretty well, CYMA VFC clones are the way to go. Hope this helped...