Notes:

Metal bushings and reshimming is required for each upgrade.
The Stubby killer is quite hard to increase its FPS because of its short barrel length, unless your going to use an extended barrel (which defeats the purpose of using a 'short' stubby).
High ROF M4's tend to missfeed, its either because of the magazine OR it needs a sector chip (sometimes both).
SP140/500fps upgrades requires a reinforced gearbox OR just wait for the stock gearbox to break (and it will break...)
SCAR and M4 variants have their own different airseal nozzle.
Battery location (and size to use) depends on M4 /M16 variants (butt, hand guard, ANPEQ, and battery bag).

Caution: Different brands of M4/M16 have different designs, some might be incompatible with other brands (some brands are not TM based).

ROF upgrade

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

Stubby length

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

M4a1 length

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 ver2 Gearbox, Metal Teeth Piston Body, Metal Cylinder head, CA ribbed type* cylinder

* Depends on barrel length

Accuracy Upgrade

Barrel length Note:
Stubby - 110mm(?)
M733 - 300mm
M4a1 - 363mm
M16a1 - 509mm

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 M4/M16. Proceed with caution.

For comments and suggestions, please post it here.


Got this from FAS... hope this helps