I have been pondering on a "super carbine" for awhile now and finally got round to building it.
With 5.56 speed is king, and the 20" does speed real well. The A5 buffer system allows a little more maneuverability and I've been wanting to try it out, as much as I like the old A1 this new/new is a nice, smooth system in it's own right.
I ended up putting the scope on another build, but I really dig the Sig Whiskey-3 it is nice glass considering the low price point.
Vltor A5 buffer kit
(Poverty Pony lower)