I have a BM 16" light barrel mounted on a Rock River flat top receiver. It has RR green M4 handguards with a Streamlight light mount rail. It will include the bolt carrier ass'y.
I can change out the flat top for a BM A2 receiver with a Mark Brown Custom "CQB A2 Mount." This has the standard A2 sights with the carry handle removed and a sight rail installed. The sights will index with an Aimpoint or other like optic. This enables you to mount an optic and still retain use of the iron sights. The new RR receivers, that resemble this set up, will not index with optics.
$450 for the flat top.
$550 for the CQB A2 receiver.