Desktop manufacture promises a potential for homemade weapons of several types, from potentially using 3D printed firearms to entirely novel designs. While policy implications have been covered before, many aspects of the field could be entirely novel relative to current technology. In other words, homemade weapons and gadgets may not be limited to ones we are familiar with today.
For example, a homemade firearm may no longer need be either a "zip gun" or some ad hoc apparatus firing conventional brass cartridges. Existing technologies hint at what may come. Caseless ammunition, for example, means that all one needs is a projectile and propellant. Desktop production of caseless ammo may mean that ammunition becomes even easier to acquire. (Quality, performance, and consistency of ammunition, however, is another matter entirely.)
One relevant topic to this is stacked ammunition, or a superposed load. This technology is an ancient one, dating back to the first firearms in history. The concept is multiple bullets in the same barrel, each ignited separately. This was done with wheellock, flintlock, and caplock systems, but this concept has been revisited with the Aussie Metal Storm concept. The Aussie concept used several bullets in the same barrel with electric current used to ignite each round. The barrels would be swapped instead of magazines changed. Imagine, in the not too distant future, a Metal Storm-like system that can be printed at home with the bullets built right in the barrel. Such a weapon would likely lack accuracy and power, perhaps compensated for by a high rate of fire.
However, the Metal Storm system and this system has flaws. For instance, the Metal Storm operates best when using low power, low recoil loads. This means that powerful, armor piercing or rifle-type rounds may suffer, but pistol or shotgun type shells may work. However, the high volume of fire is offset by the extremely low ammo capacity of the system. So, it may be more useful as a blank firing system (essentially a glorified Roman candle), rubber bullet spraying riot control device, or anti-missile point defense for warships.
As such, designs with multiple smoothbore barrels from history may come back. For example, the pepperbox pistol may return. A related design is the duck-foot pistol, where barrels are angled away from each other (such a weapon was preferred by naval officers during the Age of Sail) due to utility in close range combat. A revolver with preloaded cylinders may also return, perhaps with several bullets in each cylinder. Furthermore, each barrel (or cylinder) can be loaded with separate types of munitions. One may fire subsonic munitions in one barrel, conventional rounds in another, rubber bullets in another, and so on.