Bushmaster AR15

A friend recently purchased this rifle. It has a Bushmaster lower, and not sure of the upper. May possibly be all Bushmaster. It has a Cerro Forge upper, and pic below of how barrel is marked. Am I correct in assuming this is a Bushmaster barrel? It is chrome lined. Is anyone familiar with this configuration. Looks kinda odd to me. Never seen a setup like this. Is this something Bushmaster would have done or something someone else did? The butt stock is a Rock River & forend is UTG. It required relieving the underside of the upper hand guard to clear the gas block, which apparently is just a front sight that's been cut off. My guess is it had a fixed butt stock and a standard rifle length round hand guard when originally built.

https://www.milsurps.com/images/impo...arfclhfl-1.jpg
https://www.milsurps.com/images/impo...bd9r3agv-1.jpg
https://www.milsurps.com/images/impo...she7odug-1.jpg

If anyone has any imput or questions, they are greatly appreciated. Just trying to help out a friend with some questions I couldn't quite answer. TIA!

It sounds like something that's been assembled from various parts. "B" is supposed to be a Bushmaster marking. Here's more on this. AR15 Barrel and Bolt Stampings Information