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The Colt 606 heavy barrel was designed for automatic fire and the first attempt to use a heavy barrel on the AR-15/M16 receiver. The twist was 1 in 12 on the barrel.

I have fired this barrel at...

The first heavy barrel (1 in 12 twist)that used triangular hand guards was the Colt 606, four variations manufactured. I found this upper years ago in almost new condition.112087112088112089112090