hey guys, i'm looking to get myself into debt... i mean get myself broke 
...by putting in a few days of installing parts onto a new m4. this gun will is going to be all custom, the only stock part will probably be the gearbox from whoever manufactures the gun (unless i have to downgrade the spring to a 110). first off, looking towards a KWA, or G&G. any ideas on which models to get? money isn't an issue here, just looking to see how much i'm going to need to save up for it all. my use for it is some cqb, and mainly small desert field play. possibly going to be doing milsim, but i'd most likely be a recon/infantry man in those games. so, i guess here's the real question i'm asking: what gun(s) have the best gearbox inside them for cqb play, with a pretty high rof? think of a gunner's rifle, that's what i'm aiming for, just don't remember what the technical term is.
also, do you guys think that a better alternative would be to just build from scratch? buy the body, gearbox, etc. all separate? if so, what gearboxes are out there that offer pretty high rof, as well as quality shim jobs, and are very reliable?
Where to begin... first your desire to spend money... err shall I say waste money. Most AEG's produced today, including some of the cheap clones have the internals required to do CQB (Not sure what that FPS is, usually it's 1J or ~330 w/ .20's or some places do 350 w/ .20's) and upgrade to the Lionclaws rules or Airsoft AZ's rules (375 w/ .25's MAX and 400 w/ .20's respectivly). So gutting is completely unnecessary. As to purchasing and only leaving the gearbox left, thats excessive. Especially with M4 variants because many manufacturers produce multiple versions of an M4, usually one will fit your preference. Externals are subjective, as to what to start with?
If your goal is to do Field AND CQB in one gun you are limited to basically to the CQB FPS limit with the exception of three guns... Those three guns are the Systema PTW, Celsius CTW, and the ICS M4 series. Of which, the most practical of the three is the ICS M4. If you plan on raping it externally and replacing a lot of the externals, the ICS isnt a bad choice. The reason I say you wont be limited by those guns is because you can hot swap upper gearboxes by removing the rear body pin and pulling out the upper gearbox and replacing it. Ideally, this means you can go from your CQB setup (330 w/ .20's) to your Field set up (400 w/ .20's) with the swap of the upper gearbox. How practical that system is depends on how often you switch between CQB and Field play.
Internally, the lower gearbox on the ICS is spot on, the only thing I'd really tamper with is the addition of a MOSFET system with at least Active Braking (look at
www.extreme-fire.com for said MOSFET systems). The upper gearbox you can rape to your content, they come stock shooting from 315-330 so you can buy a spare upper gearbox and fit your "field" FPS upgrades in. Assuming you buy another complete upper gearbox to gut, I'd replace the spring guide, spring, piston, piston head, and cylinder head at a minimum, that leaves you with the stock Airnozzle, stock tappet plate and tappet plate spring, and the cylinder. To get to ~ 400 FPS, a V2 Bearing Spring guide made by Modify/Guarder/Whatever in addition to a quality piston (Prometheus Hard is always a good choice, I've had decent luck with the Bravo Piston in my M14 at 450 w/ .20's or I also had good luck with the Guarder Polycarb piston) and a G&G Vortex Piston or any other bearing piston head (Yeah yeah, you dont need bearings on both ends, but you'll see why I am recommending both in a little bit) a Guarder SP110 spring (yes, SP110 should chrono about 390 w/ .20's with both bearing piston head and spring guide) and a Modify/Guarder Cylinder head. Add a tightbore barrel (I've had excellent luck with Madbull's 6.03's) a good hop up rubber (Guarder Black or G&G Green) and an H Nub or Concave Nub (SCS) and replace the piston head O-ring with a #14 O-ring from the hardware store and you should see very consistent FPS for field use in the area of 400 w/ .20's and it should be very accurate and long ranged.
Run the sucker on an 11.1v LiPo with the MOSFET system and you should be somewhere around 20-25 RPS which is more than enough for both field and CQB use.
Externally, at a minimum you will need to replace the outerbarrel. ICS still uses poorly designed (but decently executed) multipiece barrels that after years of use will develope some flex to them. When I bought my ICS M16A3 I replaced the outer barrel imediately with a Guarder Multipiece steel barrel and that sucker went form a bleh gun to a "WOW!" gun.
Lets break down the parts list one last time:
===Internals===
MOSFET system with Active Braking (or more)
11.1v LiPo (C rating * Ah (mAh/1000) needs to be higher than 30 amps at a minimum)
Spare ICS Upper Gearbox (Stock FPS but Complete)
--In Spare Upper Gearbox--
Guarder/Modify V2 Cylinder Head
Guarder/Modify V2 Bearing Spring Guide
Prometheus Hard Piston
G&G Vortex Piston Head with Bearings
#14 O-ring
Stock ICS Cylinder
Stock ICS Tappet Plate and Tappet Plate Spring
Stock ICS Airnozzle
Guarder SP110 Spring
Madbull 6.03
G&G Green Hop Up Rubber/Guarder Black Hop Up Rubber
Big Out/Element/Echo1 H Nub or SCS Concave Spacer
===Externals===
Quality One Piece Outer Barrel
Whatever else you want to modify Externally, ICS guns have proprietary recievers so no-go on changing the reciever but everything else is good to go.
Thats my recommendation anyway. Cheers!
