I don't even think it counts as a milestone in 3D printing. It is essentially *is* a zip gun. The hard part of the finished *system*, whether it's a printed plastic gun or a zip gun, is in the ammunition. A "gun" such as this simply provides a source of mechanical impact to ignite the primer in a cartridge. A nail and rubber band can perform that trick.
When you can 3D print, on a printer that is within the price range of a consumer, a gun that approaches the reliability, accuracy, and ergonomics of a cheap handgun, that'd be a milestone.
"Most of us, when all is said and done, like what we like and make up reasons for it afterwards." -- Soren F. Petersen