The Airsoft GI Full Metal FMG4-A3 SP AEG is solid gun. I played a few backyard games and it met my expectations if not exceeded them. Accuracy was solid, tight groupings (for and airsoft gun) at about 35-40 yards. I still need to fine tune the hop-up to get it where I like it. RoF is good, could use some improvement but not much. I can't speak for the FPS, I haven't chronoed the gun yet but my friends tell me it has a bite to it. The only problem I ran into was the magwell. It was to small for any of my mags, even the one it came with was a tight fit. After 15 to 20min of sanding the magwell down with coarse sand paper, I finally got my mags to fit right.
I have not cracked open the gearbox as of yet, so I cant speak for the internals. But form how this gun has been performing, you shouldn't need to change a thing.
Solid build, no wobble
Accuracy was spot on
FPS seemed to be descent
RoF could be improved for some, but was perfect for me
Range is grate
The magwell was a problem. I don't know if i just bought some wonky mags (they fit fine on my G&G m4) or what, but GI take note.
Price: even though this is a solid gun, I question the price. I bet you can get another GI custom for less and have it be on par or even better than this one. (price could be hidden in the internals)
No front flip-up sight. More of a personal preference then a con but wouldn't hurt to include with/on the gun.