High-Speed M120 Build Kit for M4 AEG (V2 Gearbox)

The High-Speed M120 Build Kit is for you if you want an M4 AEG with lightning-fast semi-auto response and a high rate of fire, but still with enough power to keep the BB stable at range. The kit is based on a 13:1 gear set, a 30K long-shaft motor and an M130 or M140 spring, and the target we build to is around 30 rounds per second on an 11.1 V battery. With that come the high-speed modifications such a setup requires: a reinforced anti-reversal latch, upgraded bearings and a short precision barrel.

A high-speed build stands or falls on the details. When the gearbox cycles this fast, the air system must deliver full compression every time, and the barrel must not be longer than the air volume can support – which is why the barrel here is 290 mm. At the same time, high revs put far more load on bearings and anti-reversal latch than in a standard AEG. The kit is put together so the weak points are addressed from the start, and we recommend having it fitted in our workshop, where the piston is also adapted (short stroke).

High-speed gearing, reinforced parts and a matched air system

The parts are chosen so the whole gearbox can withstand the high rate of fire:

• 13:1 CNC gear set: A fast ratio that gives a markedly higher rate of fire than the standard 18:1 and faster semi-auto response. Combined with the 30K motor, it reaches a rate of fire around 30 rounds per second without having to push to an extreme motor choice.
• CNC-machined hardened steel anti-reversal latch (V2): At high revs the latch takes a hard hit from the bevel gear every time the trigger is released. A standard latch wears round at the edge and can fail under a stronger spring. The Retro Arms latch is CNC-machined from hardened steel for V2 gearboxes and comes with both a strong and a weak spring, so engagement can be matched to the build.
• 290 mm Maple Leaf precision barrel (6.02 mm): The barrel length is matched to the cylinder's air volume in a fast build. A barrel that is too long costs velocity and stability when the gearbox cycles quickly. The barrel is chrome-plated brass, and the tight 6.02 mm bore gives better compression around the BB than a standard barrel.
• Ceramic 8 mm bearings: Lower friction and better durability than standard bushings at high revs. This means less heat build-up and a more stable cycle over many shots.
• Piston with metal rack and padded cylinder head: The 14-tooth piston with metal rack and the padded aluminium cylinder head absorb the frequent impacts a high-speed build produces and protect the gearbox shell.

Maintenance and safety

• The build requires shimming and short-stroking of the piston. We recommend having it fitted in our workshop
• Use an 11.1 V LiPo battery of at least 2200 mAh, and always charge with a balance charger in a LiPo safety bag
• Measure velocity with a chronograph after fitting. With an M140 the gun may fall into the semi-only class at many sites – always check the site's rules
• Clean the precision barrel regularly – the tight bore is more sensitive to dirt and silicone than a standard barrel
• Inspect bearings, anti-reversal latch and piston after a season with many game days
• Always wear approved eye protection during play and testing