Just to clarify Kevin's post:
Armalite was the original company that began making their carbines, which require 16" barrels for non-NFA legality, with the front sight block further forward on the barrel. This obviously repositions the gas port in the barrel, and requires a longer gas tube and handguards. RRA's uppers copy this setup exactly.
Bushmaster and Olympic Arms make a different 16" upper (Bushmaster calls it the Dissipator, and Oly calls it the Eliminator) which uses full-length (i.e., "rifle) handguards, and use TWO front sight assemblies. A shaved front block is positioned UNDER the handguards in exactly the same place it is found on a standard military carbine, and is used only as a gas block. A second front sight block is positioned at the front end of the barrel where it would normally be on a 20" barrel, and is used only as a front sight.
-Troy