Notes:

* Metal bushings and reshimming is required for each upgrade.
* High ROF AEGs tend to missfeed, its either because of the magazine OR it needs a sector chip (sometimes both).
* G36 airseal nozzle is quite rare.
* Take care of your motor cage, the china G36 variants are known to have weak motor cages.
* CA G36 have small holes for the tabs of a ver3 spring guide, minor filing must be done - lekiboy

Caution: Different brands of G36 have different designs, some might be incompatible with other brands (some brands are not TM based).

ROF upgrade

Note: Battery space might be limited on some models (handguard size).

Stock : 8.4v NiMh
Lvl1 : 9.6v NiCd/NiMh or 7.4v Lipo
Lvl2 : 11.1v 12c lipoly, High Speed Motor, Mosfet, Bearing gearbox
Lvl3 : 11.1v 20c lipoly, Magnum motor, Mosfet, Bearing gearbox

FPS upgrade

G36c/k

400fps+ : SP120, airseal nozzle
450fps : SP130 or PDI 190%, Metal spring guide with bearing, piston head with bearing, airseal nozzle
500fps : SP140 or SP150, Metal spring guide with bearing, piston head with bearing, airseal nozzle, STU or ITU gears, high torque motor, Metal Full-tooth/half-tooth (for ITU) piston body
550fps : SP150, Metal spring guide with bearing, piston head with bearing, airseal nozzle, STU or ITU gears, high torque motor, Half-tooth piston body (for ITU gears), Bore Up Kit.

G36

400fps+ : SP110, airseal nozzle, sealed hopup.
450fps : SP120 or PDI 190%(470+), Metal spring guide with bearing, piston head with bearing, airseal nozzle
500fps : SP130 (490+)/ pdi260% or SP140/pdi300% (520+), Metal spring guide with bearing, piston head with bearing, airseal nozzle, TU (sp130)or STU gears (sp140), high torque motor, Metal Full-tooth piston body
550fps : SP150, Metal spring guide with bearing, piston head with bearing, airseal nozzle, STU or ITU gears, high torque motor, Half-tooth piston body (for ITU gears), Bore Up Kit.

Durability Upgrade

Lvl1 : Polycarb Piston Body
Lvl2 : Metal Teeth Piston Body
Lvl3 : DF Reinforced ver3 Gearbox, Metal Teeth Piston Body, Metal Cylinder head, CA ribbed type* cylinder

* Depends on barrel length

Accuracy Upgrade

Barrel length Note:

* 495 mm (18.9 in) (G36)
* 355 mm (12.5 in) (G36K)
* 247 mm (9.0 in) (G36C)

Lvl1 : SCS, Hop rubber
Lvl2 : 6.03 ID Tight Barrel, SCS, Hop rubber
Lvl3 : 6.00 ID (or longer) Tight Barrel, SCS, Hop rubber

Disclaimer :

1. Not all will achieve the same results, it depends on parts brand and skills of your gunsmith (or your skills).
2. Brand specification is not given, since everyone have their own brand preferences.
3. Prices depends on store/location.
4. This is for reference only, the author is not responsible for damages that might be caused to your G36. Proceed with caution.

For comments and suggestions, please post it here.

Got this from FAS... hope this helps