I have some brass with what I guess is ejection marks.Looks dented In the middle of the case.I've never reloaded for an AR .Will the fl die remove these marks or are they garbage?
Mine does the same thing on most of my brass but don't worry the sizer will remove some of the dent and the rest will not make any real difference in your guns shooting.
It is caused by the brass hitting the deflector behind the ejection port. You can put some self adhesive velcro(soft side) on the reflector to cushion it. And no, it won't cause any harm to shoot the brass.
One of the things you might do is change out to a heavier buffer and slow the bolt down, possibly change the spring (like red springs or Tubbs) out too and/or use an adjustable gas block. It sounds like you're over gassed if it's really denting the case.
Reload and shoot it. Won't hurt a thing. Even after they are reloaded and shot you will still see a smudge from each dent left by the deflector from past firings, I count the marks to see how many times the lot of brass has been shot.