I have a 2007 ICS PCR-97 M4 with a full stock, so I elected to write a review on the M16 which has more of a similar body construction to my gun. The stock can not hold a large 9.6V battery without modification like other brands, due to the split gearbox construction of the gun. I was able to modify the stock by removing several plastic cross braces inside the stock, and it now fits a 4000 mAH Elite 9.6V battery. This gun shoots very smoothly and the stock shimming is acceptable, but I recommend a re-shim as I do in all AEGs. Factory guns are manufactured the same way but the tolerances of the gearbox housing or the gears may not be exact every batch +- .02-.03mm, so a re-shim is never a bad idea. I absolutely love the split gearbox design, swapping springs to match field limits or to swap an O-Ring for improved compression has never been easier. Add that to the ICS patented gear disengagement side assist lever that releases the piston and saves your spring from long term compression, and you have an unmatched piece of engineering.
This gun is Marui compatible save for the hop-up unit, which is plastic and built to specific ICS specs, so modifications MUST be done before replacing with an aftermarket metal hop up unit. The stock plastic hop-up unit is not bad at all and many choose to just stay with it. I use a systema bucking with no fitment issues. I dropped in a 6.04mm TB barrel, Guarder Piston, Systema M120 (pulls with ease), Metal Spring Guide, and Aluminum Ball Bearing Piston Head, and still the ROF exceeds that of MPEG JG's, DBOY's, A&K's, etc. The motor is a high torque motor and will probably pull an M130, the 120 was not a problem.
The metal construction adds durability to the gun, though the hand guards are plastic and not as realistic due to the toy like gloss that other guns do not have.
I highly recommend the ICS M4/M16 series. These are absolutely phenomenal guns out of the box and are very easy to mod due to the two piece gearbox design.
High grade metal recievers
Olympic Arms Trades
Split gearbox design
Marui compatable, save for hop-up unit
High torque motor
Can hold large 9.6V if stock is modded
Forward assist is fully functioning
Handguards too glossy
Hop-up unit is plastic, not easily replaced