Windham Weaponry’s powerful 450 Thumper is available as a complete Upper Receiver Assembly – ready to snap onto your AR15 Lower Receiver/Stock Assembly. The Assembly includes a 16” .450 Bushmaster Caliber Barrel with a Windham Weaponry stamped Freefloat Railed Handguard already installed, a .450 Bolt & Bolt Carrier, charging handle and a straight, 5 Round .450 Magazine. The Barrel is heavy profile 4150 Chrome Moly Vanadium 11595E Steel, Chrome Lined in Bore and Chamber, rifled 1 turn in 24 inches – right hand twist – with 6 lands & grooves. The muzzle is fitted with an A2 Birdcage Type Flash Hider. The complete Upper Receiver is laser engraved .450 Bushmaster for safety – do not mix ammunition when changing AR rifle calibers!