C&R only pertains to the actual firearm in question being 50+ years old, or deriving its value from obscurity or rarity. Reproductions of such firearms DO NOT COUNT. This was what I last read on the ATF website last week.
Modern, 9 times out of 10, will NOT coincide with C&R eligibility.
Have the gun around when someone touches off a black powder gun. Hahah. Just let it sit in the smoke for a moment... that will get the petina on there post haste. I have a reproduction made Brown Bess flintlock, and I didn't get to cleaning the brass fittings fast enough, and they took a petina and browned up overnight. I was able to take some soft brillo to them, some metal polish, and the furntire is back to where it was, but legit, be near a flint shooter, and it'll be QUICK, bud. Hahah