Just wanna know how it works out? I’ll likely never handle one, and just wanted to know if anyone has tried this configuration.
It’s called a “dissipator” upper. Google it. 
By ‘carbine middy’ do you mean a 16" upper with a rifle-length handguard? If so, then yes. I have two 16" mig-gas uppers configured that way, and plan to do/re-do 1-2 more in the same way. I like it. Longer handguards mean you can push your support hand out farther, you can run a longer sight radius, and so on. In some circles a 16" barrel + a rifle length by is referred to as a ‘recce’ upper.
Works great. This is my 16" middy with Rifle handguards.
I suppose with a LW barrel you could use a full length rail and still handle reasonably well? Having more rail space, leaves more options for hand positions, etc.
Yes, LW Barrel and Dissy Configuration handles well
I’ve got a mid 14.5 and lightweight mid 16 both with freefloat rifle length guards.
I like them a lot, and don’t feel they are heavier or slower-handling than other setups.
If referring to the dissipator setup: https://www.m4carbine.net/showthread.php?t=62149
If referring to a barrel with some sort of lo-pro gas block and a FF rail that ends close to the muzzle device…it’s just rail of a certain length installed on a barrel of somewhat greater length; nothing exotic.
The way the title is phrased makes it a coin-toss as to which is being referred to, so…if the former, there’s a specific thread to see. If the latter, you can’t swing a dead cat in the “XXX Picture” threads alone without hitting a relevant photo/description.