The Colt 606 heavy barrel was designed for automatic fire and the first attempt to use a heavy barrel on the AR-15/M16icon receiver. The twist was 1 in 12 on the barrel.

I have fired this barrel at 100 yards slow fire using a Colt Sporter SP1 semi auto lower made in 1974. Accuracy was the same as the light barrel.

I think these Colt 606 uppers are not common as I really have seen too many over the last twenty yearsAttachment 112100Attachment 112101Attachment 112102