Don't worry, it's normal and natural. Light rub marks on top of the barrel hood are a result of gun parts in motion under regular firing conditions. It’s only cosmetic. My R9s shows the same thing and so do all the R9 pistols I've seen owned by friends of mine who actually shoot their firearms. Come to think about it, even my Wilson Combat CQB .45 ACP 1911 pistol shows this kind of wear. As long as we're talking about normal wear (not tear) and light rub marks and not gouging or galling, everything should be just fine. Just make sure the little guy is kept clean and properly lubed and you’ll be okay.