Modifying your Airsoft gun useful tips:

Many people modify there gun because they want to get more power, range or accuracy out of there gun. The next few tips should hopefully help you in your future modifications.

Gearbox versions: 

There are 9 types of airsoft gearboxes, each designed to fit a specific shape of a type of gun, for example version 2 gearboxes are for M4-M16 kind of guns, Version 6 for M1A1 Thompsons or P90s etc etc...  

Version 1: these gearboxes where the first AEG ( Automatic Electric Rifle) gearboxes. They are designed by Tokyo Marui and are used in the L85 and early Tokyo Marui FAMAS. The motor is inside the gearbox which makes it durable. The disadvantage with this gearbox is that there aren't many if not no upgrade or replacement parts. 

Version 2:  These gearboxes are quite common and very upgradable gearboxes. They are used in all M4 - M16  and Mp5 bodies. find more about this gearbox at http://www.airsoft-barracks.com/forums/topic/2376-gearbox-versions-and-compatibility-guide/

Version 3: The version 3 gearbox is not quite as common as the Version 2 gearbox but still as popular. It is very similar working to the V2 but performs differently.It is found in AUGs AK47s and G36s. 

Version 4:  The version 4 gearbox is very rare and is only found in the PSG-1. Find out more at http://www.airsoft-barracks.com/forums/topic/2376-gearbox-versions-and-compatibility-guide/ 

Version 5: The version 5 gearbox is very skinny

 and used only in the UZI. It is mainly made 

out of Plastic 

Version 6: This gearbox is used in the P90 and 

M1A1. It is quite similar with the version 2. 

Version 7:  This gearbox is used on M14s. it is very durable 

and has unique parts. 

Version 8: The V8 is only used in the type 89. It is very similar

to the V2 gearbox except it had a burst option.

Version 9: This gearbox is very new and has a completely

new design. It is used in the Tokyo Marui AK74MN. 

Barrels 

Many people change their barrel to tight bore barrels ( With a diameter of less than 6.05 mm ) They do this because the 

tighter the barrel is to the BB, the less it can bounce and move around and will therefore be more accurate. You can also make your gun more accurate by changing the length of the barrel. Here is a list of guns and their barrel length: 

110mm-TM MP5K

141mm-TM MP5-PDW

182mm-TM MP7A1

195mm-TOP UMP

208mm-TM G3 SAS

229mm-TM MP5A4*, MP5A5*, MP5SD5*, MP5SD6*, AK-47 Beta Spetsnaz (Also fits the ICS MP5 Series)

243mm-TOP MP40

247mm-TM G36C*, P90, CAR-15, SIG 552

280mm-CA SAR M41 FS Offizier, CA M15A4 CQB

285mm-TM MC-51

300mm-TM M1A1 Thompson, TM M733, G&P M4A1 Commando

304mm-G&G FS51 Fixed Stock

345mm-STAR M249 MKII

355mm-CA G36K

363mm-TM M4A1 New and Old Types, TM SR-16, TM S-System, TM XM177-E2 G&P M4A1, G&G GR-16 R4 Carbine, *(Also fits the CA M15A2 and M15A4 Carbine models), STAR M249 Para, CA M249 Para

375 mm-G&G AK104

420mm-TM SOC-16, G&G SOC-16

429mm-CA 33E

455mm-TM AK-47, AK-47S, ICS AK-74M

463mm-CA SLR-105, G&G RK-103

469mm-TM G3 SG1

472mm-TM FAMAS F1, FAMAS SV

475mm-VFC MK43 Mod 0

495mm-CA G36E, CA SAR Taktik

500mm-

510mm-TM M14

509mm-TM M16A1VN, M16A2, AUG, CA AUG A1 *(Also fits the CA M15A2 and M15A4 Full Rifle models), CA M249 MKII

534mm-TM SG 550

550mm-TM M16A2 (Long Barrel), TM M16VN (Long Barrel), TM AUG (Long Barrel), King Arms FN FAL, Guarder T-65K2

574mm-TOP M249 Series

590mm-TM PSG-1

650mm-TM PSG-1 (Long Barrel)

Barrel length: Classic Army Version

MP5K: 110mm

MP7: 182mm

G3 SAS: 208mm

MP5:229mm

MP5SD's: 229mm

AK Beita: 229mm

AK SPETZ: 229mm

P90: 247mm

G36C: 247mm

SIG 552: 247mm

M1A1: 300mm

M733: 300mm

XM177:300mm

M4: 363mm

M4S: 363mm

Knights SR-16: 363mm

M14 CQB / SOC 16: 420mm

Type 89: 433mm

AK-47/S: 455mm

G3A3: 469mm

G3A4: 469mm

FAMAS F1: 472mm

FAMAS SV: 472mm

M16VN: 509mm

M16A1: 509mm

M16A2: 509mm

AUG's: 509mm

M14: 510mm

PSG-1: 590mm

M15A4 CQB Compact: 200mm

M15A4 CQB Compact Seals: 200mm

MP5 SD's: 229mm

MP5: 229mm

G36C: 247mm

M15A4 CQB: 280mm

SLR 105U: 280mm

G36: 355mm

G36K: 355mm

M15A4 Carbine: 363mm

M15A4 Tactical Carbine: 363mm

M15A4 Railed Carbine: 363mm

M15A4 SPC: 363mm

M15 XM177: 363mm

SLR 105 A1 Compact: 364mm

SCAR-L: 384mm

SLR 105 A1 Para: 463mm

SLR 105 A1: 463mm

SAR Taktic: 495mm

CA33E: 510mm

M15A4 SPR: 509mm

M15A4 Rifle: 510mm

M15VN: 510mm

CA25: 510mm

AUG: 510mm

M249 Para: 510mm

M249 Mk. II: 510mm

SA8-2: 510mm

M15VN: 510mm 

(Copied from http://www.houstonairsoft.com/forums/show

thread.php?t=4475) 

Bushings: 
Bushings are placed on the shell of the gearbox. They are where the different gears are held in place.
They are changed if you put a more powerful spring in your gun. (Changed from plastic to metal). To do this you need to know the diameter of the bushing for your gearbox.

Version 1: 7mm Plastic

Version 2: 7mm Plastic

Version 3: 6mm  '

Version 4:  ?

Version 5:  ?

Version 6: 6mm Metal (Including two ovular ones)

Version 7: Any will fit

Version 8: Version 7

Version 9: Any will fit

You can fit different types of bushings in a gearbox. Usually the bigger the better. Above 8mm is just stupid to put on.

Inside The gearbox: 

You can change many things inside the gearbox including the:

Cylinder: If you are getting a drastically longer barrel

Piston & Piston head: This can be very useful if you get a stronger spring or you want to have better air compression in the cylinder. Another technique with pistons is called "Swiss Cheesing". This means you make small holes in certain areas of the piston. This will increase the power by a bit but will also decrease the structural integrity of the piston.

Spring: This an upgrade which many people do. Different  springs are called M120 or SP110 according to the METERS per second the project the bb at. (M120= 400fps) Modifying your spring may require you replace other parts of your gearbox which will be exposed to wear and tear. This can be very expensive.

Air nozzle: Replacing the air nozzle is a good and inexpensive way of getting more compressed air down the barrel.

Gears: You will have to get more solid gears if you buy a more powerful spring. They can be very expensive but to give more lifespan to your gearbox.

Spring guide: If you get a more powerful spring, I suggest getting a new spring guide.

Outside the gearbox: 

Barrel: Getting a new tight bore barrel is one of the best upgrades you can buy. For around 50$ You can make your airsoft gun an accuracy beast (I suggest a longer 6.03 barrel)

Mosfet: A mosfet can be useful to get a faster trigger response or if you just want the awesome three round burst function!

Battery: A better battery is a simple yet very effective way of modifying your airsoft gun. A new 9.6v battery can give your gun a higher rate of fire or give your motor more power to pull back a harder spring. Don't forget to read my post about 11.1v LiPos

Accessories: 

Scope: A new scope can be very useful for aiming and enemy recognizance. I do not suggest a more than 1.5x zoom on a AEG and not more than 4x zoom for a Sniper rifle. If a Scope says its "3x30" it means that the magnification is 3x and the size of the aperture is 30mm.

PEQ box: PEQ boxes can be useful to store your bigger battery and still make your gun look awesome.

R.I.S system: If you want to install almost any accesory on your gun you will need and RIS rail somewhere. You can buy these from prices ranging from 20$ to 200$.