In fact, quantitative changes that become qualitative changes by being large enough are the entire reason we're having this discussion in the first place! It's perfectly possible to kill a person with just your hands. "All" a gun does is make it easier... but trying to dismiss all of the legal and moral questions around guns by claiming that it's only a difference of degree, not kind, isn't going to convince very many people. A big enough quantitative change is a qualitative change.
Making this part out of wood or plastic is a days work or less for a person that is moderately handy with tools, it would be probably a run of an hour for a 3D printer with some serious post processing required, say 2 hours altogether.
And that's with an investment in a 3D printer in one case and an old drillpress and some milling bits in the other, which at the moment is the lower bar to entry.
That's less than an order of magnitude.
If you could print a fully loaded ready to use weapon in a few minutes then yes, that would be a game changer. As it stands this isn't, but it may evolve in to one.
This is the interesting discussion to have, and the one I am having. And even if the weapon itself is not ready in minutes, it's easy to imagine a world in which printing the open source AK-47 is as easy as: Visit website, search for AK-47, click print on first result, eventually refill relevant cartridges. It may only pop out hours later, but it'll still be a different world than the one we live in.
I completely disagree with anelson's characterization of the difficulty; I could make "printing a web page" look as difficult if I spelled out all the steps ("purchase paper for printer, insert paper into printer..."), but that's not reflective of the real difficulty.
Remember, Thingiverse is not version 7.2.3-final. It's 0.0.1-pre-alpha. We are not at the endgame of this technology, we are at the very, very, very beginning, and even this discussion about guns is merely one high-profile example of the sorts of questions we're going to have to grapple with when the full power of software is unleashed into the physical world.
I think this is pretty clear: making something by hand is very different from having a magic button to do it for you. "You could have whittled it out of wood!" completely misses the point.
Unless you also think the creation of high level programming languages was not a qualitative change over the days of punch cards.
