Davidson Defense Thunderbird 18-inch Mid-Gas 1:24 .450 Bushmaster AR-15 Freefloat M-Lok Handguard Rifle Upper Build Kit

Thunderbird 18-Inch Mid-Gas 1-24 .450 Bushmaster AR-15 Freefloat M-LOK Handguard Rifle Upper Build Kit

This is an AR-15 upper build kit chambered in .450 Bushmaster, built around an 18-inch HBAR 4150 CMV steel barrel with a phosphate finish and a 1-24 twist rate specific to the .450 Bushmaster chambering. The Panhead billet upper receiver with a widened .458 ejection port cut anchors the front end with a profile purpose-built for the larger bore diameter of this chambering. The 15-inch ultra-lightweight minimalist rail freefloat M-LOK handguard keeps the front end trim with a partial top rail where it counts and M-LOK slots along the body for accessories. BCG and charging handle are not included.

Key Features

• .450 Bushmaster chambering, 18-inch HBAR 4150 CMV steel barrel, phosphate finish
• 1-24 twist rate, 5-8x32 thread pitch
• Mid-length gas system with mid-length stainless steel gas tube
• Micro low-profile steel gas block, .750 inch
• Panhead billet upper receiver, .458 ejection port door cut
• AR-15 ejection port door assembly
• 15-inch ultra-lightweight minimalist rail freefloat M-LOK handguard, partial top rail, no anti-rotation tabs
• KAK .458 compensator, 5-8x32 thread pitch
• Black 5-8-inch crush washer

Perfect For

• AR-15 builders sourcing a .450 Bushmaster upper assembly without BCG or charging handle
• Shooters adding an 18-inch .450 Bushmaster upper to an existing lower with a compatible BCG and charging handle on hand
• Builders who want a lightweight minimalist handguard and a purpose-built .450 Bushmaster front end in one kit

Upper Receiver and Barrel

The Panhead billet upper receiver anchors the front end with its widened .458 ejection port cut, a purpose-built design feature for the larger case diameter of the .450 Bushmaster chambering that a standard AR-15 ejection port does not accommodate as cleanly. An AR-15 ejection port door assembly is included as part of the upper. The 18-inch HBAR 4150 CMV steel barrel brings a phosphate finish and a 1-24 twist rate to the .450 Bushmaster chamber, threaded at 5-8x32, a thread pitch specific to this bore diameter. The mid-length stainless steel gas tube and micro low-profile steel gas block at .750 inch run a well-matched mid-length gas system along the barrel for the 18-inch profile, keeping the gas timing appropriate for this barrel and chambering combination.

Ultra-Lightweight Minimalist Rail Freefloat M-LOK Handguard

The 15-inch ultra-lightweight minimalist rail freefloat M-LOK handguard surrounds the barrel and gas system along the forend, prioritizing a trim and light front end profile over full rail coverage. A partial top rail provides a solid mounting position for optics and accessories at the front end of the handguard where they are most useful, while M-LOK slots along the handguard body keep the rest of the forend open and lightweight. The handguard does not include anti-rotation tabs, which is a design characteristic of this specific unit. On an 18-inch .450 Bushmaster build, the minimalist profile of this handguard keeps the overall front end balanced and manageable in hand.

Muzzle Device

The KAK .458 compensator threads onto the 5-8x32 muzzle end of the barrel, a thread pitch and caliber designation specific to the .450 Bushmaster bore diameter. The included black 5-8-inch crush washer seats at the barrel shoulder and completes the muzzle hardware for this kit. The KAK .458 compensator is a purpose-built muzzle device for this chambering and thread pitch combination.

Follow safe handling and manufacturer instructions. Ensure compliance with all applicable federal, state, and local laws. BCG and charging handle are not included.