Both Rifles - Like Sewing Machines.
You can perform this same test repeatedly with different types of rifles, regardless of who makes it, and the results will vary every single time. Without personally looking at that rifle up close, I can't determine why the rail shifted. I seriously doubt the barrel nut came loose though. Yes, if you have an iron sight or other type of aiming device mounted to the rail, your zero will be off. I can't and won't argue that point. However, if you have a modern optic on your upper receiver like most of us tend to do, you should still be able to put accurate hits on target.
In other news, Daniel Defense offers a 100% Lifetime Guarantee against factory workmanship and material defects. If anyone owns one of our firearms and isn't satisfied in anyway, just pick up the phone and call us or shoot us an email. It's that easy and we'll take care of it.
S/F
Joe