You didn't need to buy another receiver extension, you could have corrected the first one.

Mark 1" forward of the center of the buffer retaining pin center tip in the lower receiver.  Remove the buffer retaining pin, screw the receiver extension in the extra thread needed, then work the math back off your mark to see just how much of the bottom front edge of the receiver extension needs to be faced/set back.

At the same time you are marking the bottom of the tube to remove metal from it, check the top of the tube to verity that it's flush with the top inside of the receiver threaded socket with the tube screwed in the extra turn (so it clears the back of the carrier as the upper is being lowered down to lock the upper against the lower).  If the top of the tube is proud of the top face of the lower receiver threaded socket, then make a note of how much the top of the tube needs to set back as well.


Now set back face of the tube the needed amounts, break out some birchwood casey aluminum black to reblacken the end of the tube, and now install the receiver extension.

So far, so good, but we are not out of the woods yet.

With the rifle assembled, pull all the way back on the charging handle and confirm that the face of the bolt stops about 1/4" to 3/8" in front of the back of the ejection port. If the face of the bolt stops at the back edge of the ejection port, or the bolt retracts back past the back of the ejection port (you do have the correct buffer in play, hence not a shorter carbine buffer in a fixed stock that requires the longer standard length buffer), then the front edge of the tube will need to set back even farther. Not only to index the stock while retaining the buffer extension pin, but to stop the bolt rewards travel at the correct distances as well.


As for the replacement receiver extension, upper part of it not flush with the front face of the lower receiver threaded socket, so the upper section needs to be faced/set back so the back of the carrier clears the tube as the upper/lower are mating up to each other.

I'm a mill/lathe guy since I get better control and straighter cuts (plus have them), but a file will work just fine if needed.

Just remember you may not be out of the woods, until you put the rifle together and double check the amount that the bolt face comes back against the back of the ejection port window.  At min, you want at least a 1/4" with the bolt in front of the ejection back window edge.  the buffer bumper is going to wear, and will compress as the rifle is being shot, and if the face of the bolt is at or behind the back of the ejection port window, then the spent casings get danced off the back edge of the window, with a good chance, danced right back into the action to cause a stove pipe type jam.

Nope, your good to go (just check the distance back on the bolt face from time to time to make sure that the buffer bumper has not compressed, and allowing the face of the bolt to retract back past the back of the ejection port), and about the only thing I would suggest is a bottle of Birchwood Casey aluminum black to turn the silver raw edges to black (removes all the traces of the file work, so out of sight, out of mind)..

Plus comes in handy for small touch ups on the other aluminum parts when they get scratched as well.