I used a 16" Colt 6721 barrel off a Colt AR-15A3 Tactical Carbine.The barrel is actually the correct dimension right behind the FSB you just have to machine it a little bit.
Here is my sleeve that has already been machined and parkerized.
On the Colt 6721 you have three steps in the barrel. You have to machine back from the first one.
You have to machine some material off the front on the barrel to make room for the sleeve.
Here's the barrel after it has been re-parkerized by Armacoat.
The cool thing is when it was machine the C MP still remains on the barrel.
Here it is again with the sleeve installed.
The finished upper.

Here is my sleeve that has already been machined and parkerized.

On the Colt 6721 you have three steps in the barrel. You have to machine back from the first one.

You have to machine some material off the front on the barrel to make room for the sleeve.


Here's the barrel after it has been re-parkerized by Armacoat.

The cool thing is when it was machine the C MP still remains on the barrel.

Here it is again with the sleeve installed.

The finished upper.

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