Thanks,Welcome aboard.
Haven't seen any posts regarding unusual wear on the upper around the forum so far. Are you noticing something on yours?
Could just be a gouge in the finish, but I'll wait for the pics for a better idea...
Here are a few ideas that may help...
http://www.mossbergowners.com/forum/index.php?threads/how-to-post-pictures.11193/
I don't thank that it is a clearance issue it looks like maybe a fault in the material of the upper but really don't know. I have ran about 200 rounds through the MMR without a hiccup. I followed normal barrel brake in procedure and the more it's shot the titter the groups get. I'm very satisfied with the MMR.Thanks for going through the trouble of uploading your pic I don't have a MMR but I too have not heard of anything like the pic shows! Not sure what could be causing a clearance issue but I hope you find out why!