I've bought "new & unfired" rifles that were 20 to 30 years old that still had bp residue in the barrel from the test firing at the factory which they didn't clean out, and the barrels were not damaged by it being left in the barrels.
When cleaning such a rifle for the first time I do get a little nervous about whether the residue caused any rust and pitting or not. But so far, so good. The factories must not be using full house loads but only smaller test loads to function fire the new rifles.