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

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

This is an AR-15 full build kit chambered in .450 Bushmaster, centered on an 18-inch HBAR 4150 CMV steel barrel with a phosphate finish and a 1-24 twist rate purpose-built for the .450 Bushmaster chambering. The Panhead billet upper receiver with a no forward assist design and widened .458 ejection port cut anchors the front end, paired with a 15-inch ultra-lightweight minimalist K style freefloat M-LOK handguard that keeps the front end trim and light with M-LOK slots along the forend body. A .458 SOCOM and .450 Bushmaster compatible bolt carrier group and an M4 Mil-Spec charging handle complete the upper side. The lower completion package covers the full internal lower component group, a Mil-Spec heavy-duty 6-position carbine buffer tube, a standard carbine buffer at 3 oz. and recoil spring made in the USA, and a full rear hardware group. A Mil-Spec enhanced adjustable stock with a rubberized recoil pad closes out the rear end. Everything above the lower receiver is addressed in one kit. Lower receiver 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, no forward assist, .458 ejection port cut
• AR-15 ejection port door assembly
• 15-inch ultra-lightweight minimalist M-LOK handguard, K style, no anti-rotation tabs
• .458 SOCOM and .450 Bushmaster compatible bolt carrier group
• AR-15 M4 Mil-Spec charging handle
• Complete Mil-Spec AR-15 lower parts kit
• Mil-Spec heavy-duty AR-15 6-position carbine buffer tube
• AR-15 standard carbine buffer, 3 oz.
• Standard AR-15 carbine recoil spring, made in the USA
• AR-15 M16 Mil-Spec receiver end plate
• AR-15 carbine castle nut
• Mil-Spec enhanced adjustable stock, rubberized recoil pad, black
• KAK .458 compensator, 5-8x32 thread pitch
• Black 5-8-inch crush washer

Perfect For

• AR-15 builders who need everything above the lower receiver for a .450 Bushmaster build
• Shooters pairing this kit with an existing stripped or complete AR-15 lower receiver
• Builders who want a fully matched .450 Bushmaster component set with stock included in one kit

Upper Assembly

The Panhead billet upper receiver anchors the front end with a no forward assist design and a widened .458 ejection port cut, both purpose-built features for the larger case diameter of the .450 Bushmaster chambering that a standard AR-15 receiver 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. The 15-inch ultra-lightweight minimalist K style freefloat M-LOK handguard surrounds the barrel and gas system along the forend, keeping the front end light and balanced with M-LOK slots along the handguard body for accessory attachment. The handguard does not include anti-rotation tabs and features the distinctive K style exterior profile.

Bolt Carrier Group and Charging Handle

The included bolt carrier group is compatible with both .458 SOCOM and .450 Bushmaster, addressing the platform-specific BCG requirement for this bore diameter that a standard AR-15 bolt carrier group does not meet. The M4 Mil-Spec charging handle pairs with the BCG inside the upper receiver to complete the upper interior. Together these two components finish the upper side with a fully matched and chambering-specific component set.

Lower Completion Components

The complete Mil-Spec AR-15 lower parts kit covers the full internal lower component group, including the fire control group, magazine catch, bolt catch, and all associated pins and springs. The Mil-Spec heavy-duty 6-position carbine buffer tube, standard AR-15 carbine buffer at 3 oz., and standard AR-15 carbine recoil spring made in the USA form the buffer system at the rear of the lower. The AR-15 M16 Mil-Spec receiver end plate and AR-15 carbine castle nut lock the buffer tube down cleanly at the rear of the lower receiver. The Mil-Spec enhanced adjustable stock with a rubberized recoil pad fits directly onto the Mil-Spec carbine buffer tube and closes out the rear end of the build with a comfortable shoulder contact surface and a solid adjustable length of pull.

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. Lower receiver not included.