I agree that actually taking existing stocks of ammo away from people is problematic, but they can certainly keep people from buying any new ones. They can hoard themselves. They can buy it up themselves. They can demand the manufacturers stop selling it or they can demand the manufacturers start making new ammo for which there is no gun. There are any number of ways they can do. But the point is, don't doubt their intention here.
You've always got to be smart enough to go 'Well they're going to have to bring some of their own in'. We don't want to be a monopoly where we get shoved out, it has to be symbiotic.