"As this is the gearbox that comes stock with the newer APS v3 AEGs like their AKs and the UAR, I've had a chance to tinker with it.
Verdict is...frustrated. Because this is a solid piece of work that makes spring changes and even regular gearbox assembly so much easier that is fatally crippled by a completely needless design flaw. This acutely applies to AKs, so there may not be an issue with other v3 guns.
The gearbox is designed to work with APS's blowback system, which on something like an AK, consists of a piece of metal that attaches to the charging handle and slides into the back of the gearbox like a dust cover button. This screws up the entire rear layout of the gearbox, because the slit where a typical AK dust cover button goes is now used for said EBB metal thing, and only that, because the slit is not backstopped, meaning a dust cover button can just slide out the back (assuming you find one that even fits, more below). In their AKs, APS uses a completely different button that fits onto a separate slit just above the normal slit. Not only does the placement of this slit makes it impossible for a normal button to properly fit in a dust cover, but I have yet to find any AK dust cover buttons that can even fit into the slits (tried a Marui, an E&L and 2 Cymas), as they appear to be more narrow. The only option, then, would be to use APS's own dust cover button that does work fine with and without the EBB, but APS neither sells nor includes in any gearbox packages, so the only place to find it is attached to an ASK which negates getting a separate gearbox in first place except as a replacement (unlikely because, as said before, this is pretty sturdy).
In a nutshell, if you're looking to get this for a standard AK, forget it. Until APS starts selling dust cover buttons, or better yet, starts producing these in a true standard form factor without the EBB nonsense, no quick-change for you.
"I personally really like the "quick spring change" function because it means you don't have to hold it in and shove the gb together, really makes it easier. The bearings are also good. However, I did have to modify some things to fit, such as widening the hole that the fire selector goes through, and shaving the holes the motor cage screws on to. Also, this will not fit properly into a CYMA ak47 because the top is different. (doesn't allow dust cover assembly to fit over it) Overall, the shell itself is pretty good, but it sometimes must be modified to fit.
"Fair warning, this gearbox (even though it's listed in the G36 section) DOES NOT FIT in a g36 without modification.
I bought this gearbox for a custom build in a JG G36c, and it doesn't fit. the mounting rails (or whatever they are) on the top of the gearbox shell don't allow the gearbox, fitted in the lower receiver of the gun, to fit into the upper receiver.
If you're not afraid to do some basic grinding to make it fit, feel free. The piece that goes in between the rails on the top of the gearbox also requires modification of the shell so that it will fit, allowing the upper and lower halves of the gun to be screwed together.
I still gave it a 3/5 because it's appears to be very good quality. All surfaces appear smooth and the halves fit very well together. Installation into an AK would probably be pretty painless.
So, in conclusion, nice shell for an AK build. I bought it for a G36 and was pretty disappointed.