I think pretty much all new guns these days come with spent shell casings, even tho they are "throw aways" for states like mine, who aren't anti-gun. Most guns come with just 1, but my TCM came with 4--2 each of 9mm and 22TCM. Not sure why they supplied two of each.
I don't think used gun sales require them, do they?
Some nitwit fiction writer came up with the idea that you could "fingerprint" a gun by the marks the firing pin makes on the primer or that the chamber makes in the metal of the casing. And since the greatest source of firearms information to the anti-gun movement is the detective story/movie/TV show; some legislators lapped it up like a dumb puppy drinking anti-freeze. It comes off the same baloney machine that creates the question "Why didn't you just shoot the gun out of the guy's hand?" or "Just give the robber everything he wants and he'll go away; what's wrong with that?"
You can't cure stupid but you can vote it out of office!