"If you're a support gunner and you don't own one of these, you're gonna have a bad time. This gun is a BEAST. From my experience, the gearbox is stellar quality, but it needs a few minor adjustments. I personally installed an o-ring air seal nozzle (m4 type, and yes it'll work,) 509 mm Madbull 6.03 tightbore barrel (improves accuracy (debatable) and definitely adds range,) a REAPS bucking for improved range and accuracy (needs a bit of fine tuning to shoot straight,) a steel tooth strip polycarb piston (optional; the stock one lasted me 6 freaking months while running an m120. Link to steel piston: http://www.evike.com/products/30044/,) an m120 or m125 spring (the stock spring snapped after 2 months,) a bore-up cylinder, metal cylinder head, and piston head (http://www.evike.com/products/49122,) a lipo battery (optional, but recommended,) and MOST IMPORTANTLY, IF YOU USE A LIPO, GET A MOSFET (i recommend the MERF mosfet made by gate: http://www.evike.com/products/44781)
My only complaint is that the body is made of a softer metal (zinc alloy,) so the sockets for the screws get stripped real easy. Also, when taking the stock off, don't remove the pins in the stock (it'll wear down the stock over time). Instead, remove the whole stock assembly by removing the fat pin on the back of the receiver and the threaded pin just above it. Be very careful when removing screws, and make sure not to over tighten when putting them back in. Also, you should definitely get at least one MAG brand box mag. The stock box mag features sound control, but the winding mechanism jams far too often. The only other products I'd suggest are an upper RIS (http://www.evike.com/products/25789/) and a new motor (I personally love the lonex series). Just make sure if you do get a new motor that it's short type, because it won't fit otherwise.
After that huge list of parts is installed, you'll probably have spent around 500-600 dollars on this gun, but it's well worth the money. My gun personally shoots 420-430 FPS at 22 RPS with a range of around 190 ft, weighs around 17 pounds fully loaded, and best of all the gun itself has literally never jammed on me after almost a year of use (only the original box mag has).
Great gearbox shell
Excellent gear set
Large stock = a lot of room for a battery
Great ROF with mosfet and lipo
Quick release barrel
Soft metal body construction
Absolutely NO accuracy without a REAPS bucking
Poor air seal with stock piston head
HEAVY (17 lbs when fully loaded)
Crappy box mag construction
BB usage :P
All in all a great gun after you put a little extra money in it, definitely recommend if you're a support gunner or are thinking of becoming one. I gave it 4 stars because of the mediocre box mag and the poor stock accuracy, but otherwise it's absolutely phenomenal. I absolutely love this thing!