Call Bushmaster and ask them if you can purchase the DCM Competition upper. They sold this as a complete upper for years but I dont see it as an upper-only anymore, just a complete rifle.
It has the following:
The M16A2 Dual Aperture Rear Sight is specifically modified to accept interchangeable apertures available in four sizes to suit the competitor's vision and preference... .036"; .040" (Standard); .046" or .052" diameter. Additionally, the sight can be configured for a choice of 1/2" or 1/4" Minute of Adjustment for both Windage Knob and Elevation Drum.
The Competition Front Sight is specially ground on three sides for optimum clarity, and is available in the shooter's choice of two widths (.052",or .062"). It is secured within the front sight base by a set screw to avoid the possibility of any movement once the rifle has been "zeroed".
The 20" Bushmaster Extra Heavy Competition Barrel is a full 1" diameter under the free float tube, and is precision button rifled 1 turn in 8" - right hand twist - 6 lands and grooves. The Barrel material is ChromeMoly Vanadium Steel, and the Firing Chamber is machined to SAAMI specifications. It is lead lapped and hardened to Rockwell C26 to 32. Unlike many other Bushmaster Barrels, this barrel is not Chrome lined.
In the Upper Receiver, the Bolt is carefully matched to the Barrel Extension Lugs for precise lockup, maximum strength and optimum accuracy.