For just the BCG? Just order the darn thing if you want one. Or buy a Colt 6920 as they come with one already installed.
It's legal to have M16 BCG with the AR15 except make sure that it is legal with your state and local AWB laws if any.
Colt Defense had been supplying their production firearms with M16 BCG since about Feb. 2005.
Per the BATFE letter to Colt Defense dated 2/10/2005 which allowed Colt to include M16 BCG with their production rifles:
Page 2 of 2 of the letter by Sterling Nixon Chief, Firearms Technology Branch quote:
"We can only inform you that if this installation were to create a firearm that fires automatically, it would be a machinegun as defined; conversely if it did not result in the production of a weapon that shoots automatically, it would be lawful to possess and make."