In 2018 and 2019, Arms Unlimited has been selling exclusive “MK12″ heavy barreled 18” special purpose rifles and upper receiver assemblies. You can see the complete rifle here: https://www.armsunlimited.com/Colt-MK12-HBAR-Semi-Auto-18-Special-Purpose-Rifle-p/mk12-hbar.htm and the upper receiver here: https://www.armsunlimited.com/Colt-MK12-18-inch-Upper-Receiver-Assembly-p/mk12ck-oem2.htm. I have added the available information on these unique offerings to the “Upper Receiver” section and “Rifle” sections here on The Colt AR-15 Resource. The links provided above will likely go inactive once the limited stock is sold out by Arms Unlimited.