Description from Arms Unlimited website: Never before offered to the public, the MK12 HBAR is the ultimate special purpose rifle produced by Colt. The M4 Carbine roll marked lower receiver is equipped with an A2 rifle butt stock and a specially modified carbine receiver extension offering optimal performance in this configuration. The flat top upper receiver comes with a factory installed MK12 MOD1 12-inch Knights Armament free float RAS quad rail. The barrel is Colt’s 18″ 1:9 heavy stainless steel accurized precision rifle barrel threaded in 1/2×28 with an A2 flash hider included. Utilizing a rifle length gas system and a low profile gas block. Specifications Accurized 18 Inch 1/9 twist 5.56mm NATO Stainless Steel MP tested barrel Unique direct gas operating system eliminates the conventional operating rod and results in fewer and lighter components Accommodates the full range of 5.56mm ammunition, including NATO MK262, M855/SS109 and U.S. M193 Flat top receiver allows for easy mounting of optics Caliber – 5.56mm Gas Direct System, Rifle Length Low Profile Gas Block 1/2×28 Thread Pattern Lower Receivers were marked “M4 Carbine” Sale price at the time of this posting is $1,795.00 (price is for historical reference)
I purchased an Arms Unlimited Colt MK 12 HBAR rifle on 21 SEP 2019 at $1495 from Arms Unlimited. I picked the rifle up from my local FFL. I am in the process of gathering additional information on the rifle and will publish more details soon. Here is a photo of the rifle.