I've had my M3 replaced but the cartridge tube still has this fine brown (rust?) coating. It's right under the pump latch where the tube isn't coated and bare metal is left there. So far all 3 M3 shotguns I've looked at have this. I'm not too worried as I'm keeping an eye on it and will make sure it's under control. However I'm wondering if anyone else has this problem (or if it's a problem).
Presently I have another problem with the new shotgun. The rotary bolt head doesn't always rotate to get a proper seating (i.e. you can pull the trigger and hear a click but no bang). Since this is my replacement shotgun from Benelli, I'm wondering if Benelli has some QA issues.