You're right, but the wider point is that it will eventually be cheaper and easier to download a gun, practically free in fact, and totally anonymous. I was in the Makerbot shop in Manhattan and it's aimed at kids.
Will it though? How are bespoke manufactured goods going to be cheaper than mass manufactured ones? They still require plant costs and raw materials, materials you have to buy without the economies of scale of a firm that specialises in creating arms. You can, in the US at least, buy guns anonymously, part of the problem with gun control/regulation is the already extant supply of unregulated firearms for criminals to use. There also has to be malicious intent. Type of prick who shoots someone with no good reason to, is this really going to make it easier for them to do it? Similarly I suspect governments will impel manufacturers of 3D printers, fabbing technologies to provide inbuilt safeguards, similar to how in the US at least you can't colour photocopy a banknote. There are amazing implications of these machines but I'm not sure this is a particularly good example of one.