He called Armalite and they did exactly what you said they would and pieces together a kit and he bought one.
Report back when you have tested the various parts to let us know what you discovered. I bet it will be exactly what you have been told here.
As others have said, take the Armalite kit if they can piece it together. It will be a functioning recoil system without a doubt. If there is a problem with rifle function, that will not be the problem.
I did that with about 15 pistol magazines. I knew I left them somewhere, maybe in AZ. I spent about $500 replacing them and my brother found them in a place I had looked at least twice. Oh well, you can never have too many magazines.
Welcome to the forum.
Is it a factory gun or did you/someone build it from parts? Can you post a more detailed list of your gun and its parts?
You already know this, but there many things that can affect accuracy with the barrel being one of them. ARs are different than bolt guns and large frame ARs are different than AR15s.
Diamondback has stripped uppers in stock at the moment. They also have complete uppers available.
https://diamondbackfirearms.com/shop/upper-308-stripped-upper-receiver-2-5/
Welcome to the forum. Stop by the introductions and let us know more about you.
As far as an x39 upper is concerned, it does not need, nor will it work on, a large frame AR lower.
Welcome to the forum.
Can you post some more details about the build and some pictures of the point of concern? That would help a bunch to get some suggestions.
It's great to hear of the success! Don't be a stranger just because you have a functioning rifle. This is a great hangout to shoot the sh!t and have some relaxing fun.