I'm not into the ARs, so I can't make a comment on it being normal, but I don't know why it would be. If it's denting them when it ejects them is one thing and if it's caused by a chamber issue is another.
EDIT: I googled this problem and there is actually a lot of information on the net. It is basically caused when the casing is ejected and hits a part of the receiver and they are usually in the same place on each particular gun. Bad dents may cause a problem if you want to reload them, but shallows dents will probably be fireformed out during firing and then it will have another dent upon ejection. If you can determine where they are hitting, some guys are using several pieces of electrical tape, believe it or not, to soften the contact and they claim that takes care of the denting problem.