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 two-piece M4 handguard, a .450 bolt & bolt carrier, and a 5-round .450 magazine. The barrel is heavy profile 4150 chrome moly vanadium 11595E steel. It is chrome-lined in bore and chamber and rifled 1 turn in 24” – 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 Thumper for safety – do not mix ammunition when changing AR rifle calibers!