I installed the specter lenght on my RRA 16" upper.
Very nice quality stuff, easy to install, the only thing that needs improving is the locking ring, it's hard to tighten/loosen and easy to scratch when the wrench slips...
I got the heavy one first, but thought it made everything too heavy, so I exchanged it to the lightweight one, which is perfect IMHO.
Magpul covers fit nice and snug on the rails.
This is with the normal weight tube.