BullCraft M249 / Mk.46 / PKM HPA Gearbox Shell

Drop the weight. Simplify the conversion.

The BullCraft HPA Gearbox Shell is a lightweight, purpose-built replacement for the heavy factory gearbox used in compatible M249, Mk.46, and PKM airsoft platforms.

Unlike an emptied or modified AEG gearbox, this shell was designed specifically around an HPA engine. It provides secure engine mounting, consistent nozzle alignment, organized electronics placement, and easier access for installation and maintenance.

Engine Compatibility

This gearbox shell is not limited to PolarStar engines. It can be used with any compatible HPA engine that has an M249-spec nozzle available for it.

Compatible configurations may include engines from:

• PolarStar

• Wolverine

• Other manufacturers offering a compatible M249-spec nozzle

Compatibility depends on the engine and nozzle configuration—not the manufacturer alone.

Available Add-Ons

The base product includes the BullCraft HPA Gearbox Shell only. Optional components can be added when configuring your gearbox.

M249 Nozzle

Choose an optional M249-spec nozzle based on your PolarStar engine:

• PolarStar F2 M249 nozzle

• PolarStar JACK M249 nozzle

• No nozzle

Select a nozzle if you’re converting an F2 or JACK that does not already have the proper M249-spec nozzle installed.

The F2 and JACK use different nozzles. Make sure you select the option that matches your engine.

Electronics Kit

An optional electronics kit is available for PolarStar conversions. It includes:

• PolarStar plug board

• Trigger switch

• JST wire

Select this option if your conversion requires the supporting electronics. The gearbox includes a dedicated location for the PolarStar plug board.

The electronics kit does not include an HPA engine, FCU, battery, wiring harness, airline, or air fitting.

The M249 nozzle and electronics kit can be selected independently or added together.

Platform Compatibility

Designed for compatible:

• M249 airsoft platforms

• Mk.46 airsoft platforms

• PKM airsoft platforms

• Polymer and full-metal receivers

Fitment can vary between manufacturers and production generations. Minor fitting or adjustment may be required depending on the replica.

Major Weight Reduction

The gearbox shell weighs approximately 3 oz.

With a PolarStar engine installed, the complete assembly weighs approximately 10 oz, compared with approximately 2 lb 3 oz for a fully assembled factory gearbox.

That removes more than a pound and a half of unnecessary gearbox weight from the platform.

Purpose-Built HPA Layout

The engine is held securely inside the shell to maintain consistent nozzle position and reduce movement that can contribute to feeding or alignment problems.

The shortened design also creates additional room inside the receiver for electronics. In compatible builds, an 11.1V 1000mAh stick battery and FCU can fit inside the available receiver space.

Separate access points make the system easier to service:

• Front access for the HPA engine

• Rear access for electronics and plug-board placement

Core Features

• Purpose-built HPA gearbox shell

• Compatible with M249, Mk.46, and PKM airsoft platforms

• Supports compatible HPA engines using an M249-spec nozzle

• Optional PolarStar F2 or JACK M249 nozzle

• Optional PolarStar electronics kit

• Approximately 3 oz without an engine

• Approximately 10 oz with a PolarStar engine installed

• Significant weight reduction compared with a factory AEG gearbox

• Secure engine mounting and nozzle alignment

• Dedicated PolarStar plug-board location

• Additional internal space for an FCU and battery

• Separate engine and electronics access

• Compatible with polymer and full-metal receivers

• Made in the USA

What’s Included

Your order includes the BullCraft HPA Gearbox Shell plus any nozzle or electronics options selected before adding it to your cart.

An HPA engine, FCU, battery, wiring harness, airline, fittings, regulator, tank, and other components not specifically selected or listed are not included.

Demonstration Build

The demonstration video shows the gearbox running with:

• Factory hop-up unit

• 310 mm inner barrel

• Maple Leaf bucking

• BullCraft M249 Nutsack Magazine Insert

• BullCraft Cattle Drive feed system

Confirm that an M249-spec nozzle is available for your engine before ordering. If you’re unsure which nozzle or electronics configuration your conversion requires, contact us before purchasing.