New guns are machined from steel, machining uses a mix of oil and water to lubricate and flush out chips. The oil will stick to the metal, and it's only lightly filtered at best.
Further processing may include lapping, which could leave remnants of paste compound on parts.
EPA is pretty stringent about volatile petroleums and their use, I no longer expect bright shiny superclean gun parts coming out of American factories. Most of the early solutions used in production worked well, were carcinogenic, flammable, and now highly restricted. I doubt that's powder residue - especially a bolt gun. The chamber isn't opened until the barrel is fully vented, unlike a self loading action. It's likely just tool oil, and being in a recessed area, simply needs some TLC to remove. Production schedules simply don't allow for a white glove cleaning before a gun is boxed and shipped.
It's just another reason we don't have as much production here, we demand bright shiny and new, but we don't want to be the guy dying at age 52 from exposure to cleaning solvents from 25 years working at the plant.