The lack of ammo is the Achille’s Heel of firearms. Tried to buy any lately? Those 3D printers will never safely replicate brass casings. Or primers. I shoot muzzleloaders and can cast lead balls and even make my own black powder, but I and most people don’t have access to the chemicals to manufacture primer compounds; that is dangerous as hell anyway. The best I can do is chip flints for my flintlock.
Give them a bit of time and perhaps the caseless ammo propellant can be 3D printed.