Seriously, I've seen a LOT of guns butchered with a Dremel, but if you use the right bits, no problem.
Polishing a feed ramp first takes place with one of the abrasive-impregnated rubber bits, NOT repeat NOT a grinding wheel!!
Final polishing is done with a felt wheel charged with a polishing compound like Simichrome.
I check the mags out, too . . . I had some new FAL mags that had a sharp edge on the bottom of the feed lips that dragged on the loaded rounds; breaking the edges made for slick function.
So . . . when I get a new gun, I thoroughly inspect it, buff it where I think it needs it, clean & lube, then go to the range. If it then doesn't work right, I'm not a happy camper.