No real reason I can state, but my gut feeling is that you will get failure to chamber problems or extract issues. I would suspect that the generous military barrel chambering might create enough small dimension differences to cause problems. A commercial quality barrel with a well polished chamber might make a difference. I have never had luck with neck sizing except with bolt guns with single load slow fire, and I small base resize for my ARs.
Yeah, I use small base dies for my AR reloading and have always been pretty much operating under the premise that tolerances with the AR won't permit neck sizing. I also use a Wilson drop in gauge to check everything and have had Zero failures to feed/extract.
For neck sizing to be effective, the case neck needs to be turned to consistent dimensions or you are just wasting time.
Reason being, if the case neck is thicker on one side than it is on the opposite side, you induce run out and the bullet enters the rifling offset to the centerline of the bore.
Cases without uniform neck dimension will have varying amounts of tension holding the bullet. This results in different chamber pressures and more velocity spread.
The AR platform is not like a bench rest rifle and does not have the tighter tolerances due to the necessity of the manner in which the rifle is designed to function.
As long as the case neck is turned and the proper tension to hold the bullet is there, as well as sizing the body of the case so that rounds will feed properly, neck sizing is an option. Question is, is all that extra work worth the end result? In my opinion, probably not.
With inconsistent neck thickness, there is a tendency for the expander ball to , for want of a better word, drag the neck toward one side inducing run out.