I have a 20" heavy barrel on my AR and I`m looking to turn down the barrel to the rear of the sights to the diameter of military specs and I`m looking to shorten the barrel to 16"....Has anyone done that...is there any drawbacks to doing it....Actually I`ll be changing it to carbine length with a rifle length gas system...

I`m looking to make a lighter AR.....My gun runs perfectly..and.it`s very accurate at long yardages...but I just want a lighter gun...I have more interest in shooting at under 200 yds anyway...

QUESTION.... Is there any recommendations of where to send it to have the work done....Jim

Better off buying a new barrel.

+1...but you could send it to ADCO

ADCO, check their website for instructions.  

Although, going with a new barrel may be the preferred option.

There's a good chance that will change after you make the modifications you're talking about.  By moving the muzzle so close to the gas port you'll be reducing the dwell time of the system significantly, inviting reliability problems.

The costs of your modifications will also come close to the cost of a new barrel, so unless there's something very special about your current barrel, you might want to rethink your plan.

ADCO, cut and re-crown to 18", turn down the barrel in the middle. Winning. I might have to do this too with my own 20" HBar.