Yes, it is, as it is already registered and tax stamped as an SBR.
Yes
Only if there was one made for an AR-15 upper, which there isn't.
1/2 28
It depends.. if you have a carbine length barrel and will be using it on a rifle with a carbine length barrel, then yes. If you took it from a rifle with a rifle length barrel, and are putting it on a rifle with a carbine length barrel, then it won't function properly.
18 for a Title 1 rifle (16" barrel minimum, semi auto only), 21 for a pistol or NFA restricted variant (full auto, barrel shorter than 16" on a rifle lower).
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.
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If it is one of the recent Colt .22LR AR15 rifles, then the answer is no - they are completely different internally.
1959
Yes
The US Army presently uses the M4 carbine, a shorter and much modified version of the Armalite AR15.
Colt has not offered a AR15 in 308