Bull Craft M249 / MK46 HPA Gearbox

The Bull Craft M249 / MK46 HPA Gearbox is the first purpose-built HPA gearbox shell engineered specifically for PolarStar-powered M249 and MK46 platforms. Unlike converted AEG gearboxes, it was designed from the ground up for HPA performance, improved alignment, easier maintenance, and significant weight reduction.

For years, HPA M249 builds have relied on modified AEG gearboxes that were never designed around an HPA engine. The result is often unnecessary weight, compromised alignment, difficult maintenance, and limited room for electronics.

The Bull Craft M249 / MK46 HPA Gearbox was engineered exclusively as an HPA-specific solution. By eliminating everything an HPA system doesn't need, we've created an ultra-lightweight gearbox that improves engine alignment, simplifies installation, and makes maintenance easier than ever.

A complete Bull Craft gearbox with a PolarStar engine installed weighs approximately 10 ounces, compared to approximately 2 lb. 3 oz. for a fully assembled OEM gearbox. The HPA gearbox shell itself weighs just 3 ounces while providing a rigid, precision mounting platform for your PolarStar engine.

Precise engine alignment was one of our primary design goals. The engine sits securely within the shell to maintain consistent nozzle alignment and eliminate movement that can contribute to feeding inconsistencies and alignment issues. Dedicated mounting provisions for the FCU plug board ensure clean installation and proper placement without improvising mounting solutions inside the receiver.

The shortened gearbox design also creates valuable internal space, allowing an 11.1V 1000mAh stick battery and FCU to fit neatly inside the receiver while maintaining a clean, serviceable setup.

Whether you're building a competition-ready support weapon or upgrading an existing M249 or MK46, this gearbox delivers the strength, precision, and serviceability expected from a purpose-built HPA platform.

Unlike traditional M249 conversions that reuse heavy AEG gearbox shells, the Bull Craft gearbox was engineered specifically for HPA operation. There are no unnecessary gear train components, oversized castings, or compromises required to adapt an AEG design. The result is a lighter, stronger, and easier-to-service platform built specifically for modern PolarStar HPA systems.

Key Features

✅ First Purpose-Built HPA-Specific M249 / MK46 Gearbox Design
✅ Designed and Manufactured by Bull Craft Industries
✅ Approximately 10 oz Fully Assembled with PolarStar Engine Installed
✅ Lightweight 3 oz Gearbox Shell
✅ Significant Weight Reduction Compared to OEM Gearboxes
✅ Secure Engine Mounting for Precise Nozzle Alignment
✅ Dedicated FCU Plug Board Mounting Location
✅ Shortened Design Creates Additional Space for Battery and FCU
✅ Front Service Access for Engine Maintenance
✅ Rear Service Access for Electronics and Plug Board
✅ Eliminates Common Alignment Issues Found in Converted AEG Gearboxes
✅ Compatible with Polymer and Full Metal M249 / MK46 Platforms
✅ Made in the USA

Compatibility & Installation

✅ Uses PolarStar #3 M249 Nozzle
✅ Compatible with PolarStar HPA Engine Systems
✅ Designed for M249 and MK46 Platforms
✅ Compatible with Polymer and Full Metal Receivers

Additional Information

The demonstration video below was filmed using a stock hop-up unit, 310mm inner barrel, Maple Leaf bucking, and a Bull Craft M249 Nutsack Magazine Insert featuring the Cattle Drive™ feed system.

⚠️ Some photos may show markings referencing M60 and PKM fitment from prototype testing. Production M249 / MK46 gearboxes will not include those markings. Updated product photos are coming soon.