I got a brand new Interarms AR-15, but quickly discovered that many of the cartridges would not seat and would actually get stuck in the chamber. I did measure the neck sizes, and they were on the large side, but fed fine through my Mini-14.
After polishing the chamber with some automotive rubbing compound, I was able to get most of the rounds to feed properly. Hopefully with more shooting, the chamber will loosen up some more.
If anyone has any suggestions, please post a comment.