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Bayonet holder, flash hider.
Colt has not offered a AR15 in 308
If it is one of the recent Colt .22LR AR15 rifles, then the answer is no - they are completely different internally.
Colt made a special collar that slid on the barrel about 1.5 inches behind the barrel. The collar had cut to limit the cant of the bipod and was held by a roll pin. The version I saw was on a special HBAR barrel (.750 dia.). The standard M60 bipod fit between the collar and the flash hider.
You will have to call Colt and tell them the serial number.
Call Colt and they will tell you.
Proofhouse.com. Call Colt also.
@ 40 inches.
The AR15 series
Proofhouse.com has Colt sn data.
You will have to call Colt and provide the serial number to find out.