This seems like a really good project but I don't understand how the control interface works, the rest seems straight forward enough and I would love to build/have my own 3D metal printer.
My first thought was that I could make parts for the classic cars that I collect and I imagine others reading this will have their own needs and wants and desires. I don't think many people will be making guns with a printer, cheaper to just go out and buy one.
I also can see that there will be the normal amount of finishing on a printed part as when compared to a cast metal part, i.e. grinding, sanding and so on to achieve and acceptable finish.
In the not too distant future I can see that instead of buying a physical item we will download the items file and print it ourselves, this would be a huge saving in freight costs and be much better for the environment.
Dan

I imagine there will come a time when the personal ownership of firearms will be illegal, for good or for bad. In those circumstances, one assumes that this technology will only be legally owned by those with a licence to do so and draconian penalties for non-compliance.
We know, thanks in part at least to the efforts of Mr Snowden, that the internet is no longer free, so any attempt to obtain plans for firearms and the like will be spotted by algorithms constructed for that purpose. Whilst one can say that 'where there is a will, there is a way,' the numbers produced will be very small and their production short lived. Since 9/11 there is no doubt that some countries have in place all the legal instruments they need to come down hard on any transgressors on the pretext that they are probably 'terrorists' with no habeas corpus in some countries, just to show how civilised they are.
I imagine that there is a really good business opportunity here. Set up a small workshop equipped with these devices and offer to make unique parts to order. Google or the like might set up a 'cloud' where blueprints can be stored for anything and everything, appended where possible with the instructions for 3D printers to make them.
I hope, but if past experience is any guide, doubt, that all the software to drive these devices is working to the same standard so that any of them can use the same instructions. It will all depend on whether there is a progress prevention officer, sorry, accountant, involved. (Sorry, I am just fed up with being tied to one operating system because I have lots of software that will only run on it! Grrrh!)

"In the not too distant future I can see that instead of buying a physical item we will download the items file and print it ourselves."
It logically flows from this ambition that companies like HP or Epson will soon be selling overpriced cartridges of (for example) Cell Phone Paste.
Hopefully this reveals the impracticality of your ambition in the "not too distant future".
There's no denying the usefulness of 3D printing technology, but extrapolating the hype as you've just done is very naive.

What this actually does is use up expensive welding wire to produce a rough machining blank that has to be further processed using conventional grinding or machining before it is useful.
It would be cheaper, and no harder for the garage hobbyist, to produce the same rough blank by sand casting. The melting would use up far less electricity, and you would have a wider choice of material, not to mention a far higher casting quality.
The real barrier to producing homemade metal parts is in high-precision finishing, not in producing blanks. It appears the developer has eventually realized that they have been following a dead-end street, which is why they open-sourced it. "Hey, we can't think of a way this can be made profitable or useful, so yeah, you can have it".

As someone with a marketing background, the potential I see for 3-D printing (either plastic or metal) will be an increase in the level of customization that will become available for off-the-shelf type products. Previously the high cost of custom tooling, like stamping dies, and machine down-time required for set-up to run unique parts has kept consumer products limited to items with high market volume. Now 3-D printing makes it possible to produce small volume production grade parts at competitive prices. In the future we might order products on-line giving manufacturers specific requirements for ergonomic or appearance features. Imagine ordering a new car and customizing the dashboard design and layout to meet your individual taste and needs.

It may be cheap and easy to buy a gun in the USA instead of printing one, but in most other countries it's not.
In fact, in most other countries it's very difficult and costly to buy a gun due to all the red tape. And buying a hand gun is even harder and more costly than buying a rifle.
That makes 3D metal printers a serious concern in most countries other than the USA.

A great step forward in printing. for their prototype they should consider adding sand as a support media for the liquid metal that is produced. The sand could be applied a grain or two above the desired location of the metal. Melting the metal will displace the few grains of sand occupying the spot. They also have a problem with a ground. Plasma welder may fix that. Look into the research to use a laser welder to add material to shafts. the issue is preventing the metal drops from flowing away which is your big problem. They may have found solutions.

Star trek rules. This one show has and still is changing the world for the better.

So it's essentially a CNC MIG welder laying down a series of thin pad welds, Is that right?

The people here have shown there are problems, but let's face it, it is a 1st Generation version that a garage tinkerer can afford. Consider how the 3D plastic printer started, people will take the idea and run with it, improving with each innovation. 3-5-10 years, who knows what will become of plastic or metal printers?