Here are all the gearboxs available in airsoft, this little summary can help you for possible custom use.
Liste des gearbox trouvables sur le marché de l’airsoft et leur utilisation possible pour des customs.

V1 Gearbox :
(Famas)

Suitable for : machine gun.

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V2 Gearbox :
(MP5, G3, M4 and derivate)

Suitable for : small machine gun.

V2 Next gen :

SR-25 variant :

ICS variant :

Masada variant :

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V3 Gearbox :
(AK, AUG, G36, SIG, MP5K)

Suitable for : small machine gun.

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V4 Gearbox :
(PSG Marui)

Suitable for : small machine gun.

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V5 Gearbox :
(UZI Marui)

Suitable for : machine gun, small machine gun, rifle.

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V6 Gearbox :
(P90, Thompson)

Suitable for : machine gun, small machine gun.

Thompson M1/M1928 variant :

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V7 Gearbox :
(M14, Sten)

Suitable for : machine gun, small machine gun, rifle.

G&G variant :

AGM Sten variant :

Guide de démontage de la gearbox V7/M14
V7/M14 gearbox disassembly guide
Click here

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V8 Gearbox :
(Type 89)

Suitable for : small machine gun.

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MP40 Gearbox :

Suitable for : machine gun, small machine gun, rifle.

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M3A1 Gearbox :

Suitable for : machine gun, small machine gun, rifle.

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PPSH Gearbox :

Suitable for : machine gun, small machine gun, rifle.

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SVD Gearbox :

Suitable for : machine gun, small machine gun, rifle.

Real Sword SVD variant :

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BAR 1918 Gearbox :

Suitable for : machine gun.

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M249/M60/MK43 Gearbox :

Suitable for : machine gun.

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L85 Gearbox :

Suitable for : machine gun.

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T97 Gearbox :

Suitable for : machine gun.

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ECSMG Gearbox :
(Scorpion VZ61, MP7, UZI Well, Mac10)

Suitable for : machine gun, small machine gun, rifle.

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AEP Gearbox :
(G&_C, USP, P226, Desert eagle, 1911, M93R)

Suitable for : machine gun, small machine gun, rifle.

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Oioi.