Davidson Defense Mountain Key 18-inch Mid-Length 1:24 .450 Bushmaster AR-15 KeyMod Rifle Full Build Kit

Mountain Key 18-inch Mid-Length 1:24 .450 Bushmaster AR-15 Freefloat KeyMod Rifle Full Build Kit

The Mountain Key is a .450 Bushmaster AR-15 Full Build Kit built around an 18-inch mid-length gas platform. The kit centers on an 18-inch Sepal HBAR profile 4150 CrMoV steel barrel with a phosphate finish, 1:24 twist rate, and 5/8x32 thread pitch, paired with a KAK .458 compensator and a black 5/8-inch crush washer. A micro low-profile steel gas block at 0.750 diameter and a mid-length stainless steel gas tube are included on the front end. The Panhead billet upper receiver features an enlarged .458 ejection port door and an ejection port door assembly, paired with a 15-inch Trinity Force ECHO freefloat KeyMod handguard. A complete .458 SOCOM and .450 Bushmaster bolt carrier group and a Mil-Spec AR-15 M4 charging handle complete the upper side. The lower hardware includes a complete Mil-Spec AR-15 lower parts kit, a Mil-Spec heavy-duty 6-position aluminum carbine buffer tube, a standard 3 oz AR-15 carbine buffer, a USA-made standard AR-15 carbine recoil spring, a Mil-Spec AR-15 M16 receiver end plate, and an AR-15 carbine castle nut. The SOPMOD style collapsible stock in black with integrated storage compartments and QD mounts rounds out the kit. Lower receiver not included.

Key Features

• 18-inch Sepal HBAR profile 4150 CrMoV steel barrel, phosphate finish, 1:24 twist, 5/8x32 thread pitch, .450 Bushmaster
• Mid-length gas system
• Mid-length stainless steel gas tube
• Micro low-profile steel gas block, 0.750
• KAK .458 compensator, 5/8x32, black 5/8-inch crush washer included
• Panhead billet upper receiver, enlarged .458 ejection port door, ejection port door assembly
• 15-inch Trinity Force ECHO freefloat KeyMod handguard
• Complete .458 SOCOM and .450 Bushmaster bolt carrier group
• Mil-Spec AR-15 M4 charging handle
• Complete Mil-Spec AR-15 lower parts kit
• Mil-Spec heavy-duty 6-position aluminum carbine buffer tube
• Standard AR-15 carbine buffer, 3 oz
• Standard AR-15 carbine recoil spring, made in USA
• Mil-Spec AR-15 M16 receiver end plate
• AR-15 carbine castle nut
• SOPMOD style collapsible stock, black, integrated storage compartments, QD mounts

Perfect For

• Builders sourcing a complete .450 Bushmaster AR-15 full build kit with an 18-inch Sepal HBAR phosphate 4150 CrMoV barrel, Panhead billet upper receiver with enlarged .458 ejection port, KAK .458 compensator, 15-inch Trinity Force ECHO freefloat KeyMod handguard, platform-specific BCG, and SOPMOD collapsible stock with integrated storage compartments and QD mounts already included, pairing it with their own lower receiver
• Shooters building an 18-inch mid-gas .450 Bushmaster rifle platform who want the .458 SOCOM and .450 Bushmaster BCG, charging handle, complete lower parts kit, and SOPMOD collapsible stock covered in one kit with a Panhead billet upper and Trinity Force ECHO freefloat KeyMod handguard
• AR-15 builders who want a .450 Bushmaster full build kit with a platform-specific BCG, KAK .458 compensator, and a SOPMOD collapsible stock with integrated storage compartments and QD mounts without sourcing components separately

Barrel and Muzzle Device

The 18-inch Sepal HBAR profile barrel is chambered in .450 Bushmaster and constructed from 4150 CrMoV steel with a phosphate finish. The twist rate is 1:24 and the thread pitch is 5/8x32, which matches the included KAK .458 compensator. A black 5/8-inch crush washer, a micro low-profile steel gas block at 0.750 diameter, and a mid-length stainless steel gas tube cover the front end gas configuration on this mid-length platform.

Upper Receiver and Handguard

The Panhead billet upper receiver features an enlarged .458 ejection port door sized for the .450 Bushmaster cartridge and includes an ejection port door assembly. The 15-inch Trinity Force ECHO freefloat KeyMod handguard features KeyMod attachment points along the body for accessory mounting. These two components define the exterior structure of the upper assembly on this AR-15 platform.

Bolt Carrier Group and Charging Handle

The complete bolt carrier group is compatible with .458 SOCOM and .450 Bushmaster platforms. A Mil-Spec AR-15 M4 charging handle is included to complete the upper component set.

Lower Parts Kit and Buffer System

The complete Mil-Spec AR-15 lower parts kit covers the internal lower receiver components. The buffer system includes a Mil-Spec heavy-duty 6-position aluminum carbine buffer tube, a standard 3 oz AR-15 carbine buffer, a USA-made standard AR-15 carbine recoil spring, a Mil-Spec AR-15 M16 receiver end plate, and an AR-15 carbine castle nut. The SOPMOD style collapsible stock in black features integrated storage compartments and QD mounts, pairing with the included Mil-Spec carbine buffer tube and covering the rear stock interface for this build.

Follow safe handling and manufacturer instructions. Ensure compliance with all applicable federal, state, and local laws. Lower receiver not included. Verify compatibility with your specific .450 Bushmaster lower receiver before use.