I’m looking at buying one of the Colt MK12 HBARs from Arms Unlimited. In the process of trying to learn more about it, I came across this article done by the good folks at http://www.looserounds.com. The article discusses the CR6724 Colt Accurized Rifle which has a 24″ 1/9 stainless steel barrel.
The Arms Unlimited MK12 rifles have stainless steel 1/9 barrels. Knowing that these rifles were made exclusively for Arms Unlimited, I have the feeling that the 18″ barrels on the AU MK12 are left over barrels from the Colt Accurized Rifle. Anyway, I hope to have details for you very soon as work on finalizing the purchase of one of AU MK12 rifles. In the meantime, check out Looserounds article on the old CR6724 rifle here: http://looserounds.com/2017/11/01/colt-accurized-rifle-hbar-elite-cr6724-part-1/