3D printed guns made practical? Most 3D printers available today (especially at a price that ordinary people can afford) use some kind of thermoplastic material to make objects. Guns made from plastic are challenging, to say the least – plastic just isn't strong enough to withstand the dynamic forces at play in containing an explosion, which is exactly what a gun does.
a special kind of bullet that essentially contains a “disposable barrel” in the shell (some of this ammunition is at right). This is something that could easily be outlawed, but ... it also wouldn't take a sophisticated machine shop to make it, nor would it take a lot of skill. An individual or group who badly wants to make a weapon is going to be able to do it this way, with parts, machines, and materials readily available today.
There's another approach I've read about that I suspect has even more promise in the long run: to create a gun-like weapon that uses tiny rockets instead of conventional ammunition. By switching to a rocket, the need for a strong, heat-resistant barrel is eliminated. The major disadvantage to using a rocket is accuracy – but that assumes that the rocket is unguided. Off-the-shelf technology can be used to create a guided rocket today, though the cost is fairly high (over $100 a round). But those costs are coming down very rapidly, for a host of reasons. I'd bet that within 5 years one could make a 3D printable, guided-rocket weapon with ammunition that cost under $20 a round – and was deadly accurate to very long distances, and was capable of carrying incendiary or explosive warheads.
When such weapons are available to ordinary citizens, governments are going to have a much more difficult time imposing their will on an unhappy citizenry. We're already seeing a bit of this at work, mainly in the Middle East. There are many reports of social media (especially Twitter) being used to coordinate rebel military campaigns. Drones you can buy from Amazon are being used by rebel forces for reconnaissance. I've read two reports that those same drones have been equipped with small warheads and used as weapons. Radio-controlled planes are being used by Hezbollah and the Syrian rebels, and probably others as well.
It's interesting to ponder what this development means in the long run. I'm inclined to think that it's mainly bad for thugocracies (like the Assads in Syria), but good for democracies (because it will clarify the thinking of politicians if they know there is a limit to how much they can piss off the public). Clearly, though – just like any other weapons – evil people will be eager to jump on these as new tools for their trade...