Reach Under
I find that a feed ramp will not put any kind of scratches on brass. Especially a well polished one like I have done to all mine. The dings your getting are probably from the ejection port mouth or from the brass deflector.
I also polish/smooth/contour the underside of the lips of my mags. Take one of your empty mags(works best if the follower and spring are taken out),take your pinky finger and run it under the feed lips of your mags(careful not to cut yourself),you'll probably find them sharp. This will put lines on the brass as it is stripped from the mag and jammed into the chamber.
Remember that the entire process of stripping,feeding,extracting,and ejecting,is a very violent one. It is quick and forceful. It is not designed to treat brass gently. BUT......I have brass(LC72 Match) that has been reloaded 15 times. It still holds a bullet(50grBerger)that when fired will go into the same hole as the last bullet at 100yds.
"Minor" dents,dings,and scratches,are not a big deal and will not influence the accuracy OR reliability of your AR.
Good Shootin' -----pruhdlr