Armalite manufactures mid-length h.guard uppers also.  The gas block is under the h.guards and is set at normal carbine length.

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

Quoted: Are there any problems inherent in the Dissipator type carbines with the gas block under the full length handguards?

Not at all.  The gas block is in the same place as on most 16" carbines, so it functions identically.  The main advantages are:

- Rifle-length sight radius (though this is obviously only important with iron sights)
- You're less likely to burn yourself on a hot barrel, as most of it is covered.
- The barrel is a bit better protected.

The main disadvantage is the extra weight from the second gas block.  But you can solve that by getting the Bushmaster "M4 Dissipator" barrel, which I highly recommend.



-Troy

Quoted: the Main reason for the m4 setup was for the M203 I beleive, Anyone?

That and weight reduction, yes.

-Troy