Bushmaster had a contract for the M4 carbines back in 1989/90 till Colt made waves . Most went to the SEALs, CAG(Delta) and the US State Department Security Forces. The barrels are different then the current production models. They were refered to as the "wasp" barrels( I'm guessing it's a reference based on the barrel looking like a wasp's body. Early Colt 723/727 barrels were also made in this profile.
The Bushmaster carbines I've seen have the Colt Aluminum or fiberlite M4 stocks in place of the Bushmaster fiberlite stocks. I've heard but not confirmed that before being issued the following parts were replaced at Crane:
Soldier of Fortune had a write up about Bushmaster's M4 back in 1991