If you follow 3D printing news, or just geeky technical news in general, you've probably seen mention of the "new bullet" that will "make 3D printed guns a reality". Sadly, as with most reporting about the intersection between 3D printing and firearms, the reporting on this topic has been long on emotion and quite short on information. Let's change that.
I should disclaim that although I know something about 3D printing, guns and mechanical design, and I believe I'm reasonably able to read and understand things as varied as dimensioned drawings and firearms laws, I am not an engineer, gunsmith or lawyer. Take anything I might say here with the appropriate serving of salt.
First, let's dispense with the immediate inaccuracy: the .314 Atlas, which is the name given to the project by its creator, Michael Crumling, is not a "new bullet". The bullet is the bit that goes downrange; that is, the thing that the gun shoots out of its barrel. In a modern, breech-loading gun the bullet is combined with three other parts in order to make a round or cartridge of ammunition: the case, the primer and the powder. What Mr. Crumling has designed, and deserves full credit for, is an innovative case. The bullets appear to be ordinary lead balls, the primers and powder appear to be ordinary commercial parts. In fact, calling the .314 Atlas a cartridge design might be very important, as I'll get to a little later.
To understand what the innovative part is, you need to know something about ordinary ammunition. The case is typically made of brass, with a fairly heavy base and thin walls in the form of a tapering cylinder that's open at the top. The primer fits into a machined recess at the bottom, the bullet in the open top, and the powder is contained inside. The Wikipedia page has plenty of pictures and a couple of cutaway drawings. The case can be made of brass because even though the powder is going to burn inside of it, it doesn't have to contain the pressure. When the round is loaded for firing it is pushed into a carefully machined chamber at the back of the gun, a part known as the receiver. It can be a separate piece of metal, or integral with the barrel. The shape of the chamber precisely matches the shape of the case, with the bullet extending just into the barrel and the primer end of the case flush with the edge of the receiver, or protruding just a little (so the ejector can grab it, but that detail isn't important). When the round is fired, a metal pin crushes the primer to ignite it, the primer ignites the powder, the powder burns into hot gas at very high pressure which pushes the bullet down the barrel, and the bullet goes downrange. Afterwards most cases can be reloaded and reused, though some are designed to be disposable.
Of course, where the case is light and relatively weak, the receiver and barrel are heavy and strong. Machined out of steel, they can withstand the pressure of the hot gases thousands of times. The barrel also contains spiral grooves called rifling that cut into the softer bullet and cause it to spin, which dramatically increases accuracy. In the process, the bullet is formed to the barrel so that the hot gas doesn't leak past it; any leakage means lost energy.
Readily available 3D printers, the kind that use Fused Filament Fabrication (FFF) with spools of plastic filament, can't make strong receivers or barrels. The best they can do is make big blocks of plastic that will hopefully not shatter the first time that a cartridge is fired inside them, and even then are only able to handle a small, low-powered round.
Now let's look at the gun that Mr. Crumling designed to go with the .314 Atlas cartridge. He chose to use a manufactured trigger, its associated parts, hammer and firing pin. That's not important; those things could be 3D printed as well, at least the trigger parts; in his design a heavy hammer and stiff spring may be necessary, and those might have to be metal. But that's a minor detail. The key is the case design, which replaces the thin brass with heavy steel. Effectively, what he's done is combined the cartridge, receiver and barrel into one unit, each of which fires one shot (though his design is readily reloadable after it is used).
I have no doubt that this is an innovation. However, it is also a compromise in several important areas. I'll try to explain everything that I've observed so far; keep in mind that there may be workarounds for some of these complaints, but also that other issues might come up as more experience is gained with the new design.
First, the feature touted for most 3D printed guns is that they can be made entirely out of plastic, and therefore be difficult to detect. That's never actually been true; although the gun can be plastic, the ammunition still needs to have metal in it. But since the ammunition can be brass, copper and lead, it may be more difficult to detect that the typical steel gun. Clearly that will not be the case for this design, since the ammunition will itself contain substantial amounts of steel. I'm also doubtful that the hammer and its spring can be plastic, because of they way the gun works. When it is fired, the hammer briefly holds the cartridge in place, until the bullet has exited, and then the cartridge is "ejected"; really, it is allowed to fly out of the top of the gun. I don't think that a plastic hammer and spring would have nearly enough force to keep the cartridge in place, and it would likely move even before the bullet was out, an obviously dangerous situation. Of course, the hammer and spring should be easy to obtain and this is not an objection for the ability to manufacture the gun, only to conceal it. Those of us who would prefer that guns be detectable will view it as a feature.
Second, this gun doesn't really have much of a barrel. The part of the Atlas case that extends beyond the bullet as it is seated prior to firing is the only effective barrel, and it looks to be about an inch based on this drawing. Short-barreled guns are nothing new. As with anything they have both strengths (small size, light weight, lower cost) and weaknesses. They're substantially less accurate, for at least three reasons: the short barrel doesn't do as good a job of getting the bullet spun in its rifling, the light weight means the gun will be less steady when aimed, and the short distance between front and rear sights makes aiming less precise. They also don't impart as much energy to the bullet, because the hot gases from the burning powder don't push on the bullet very long, and the powder doesn't have enough time to burn before the bullet leaves the barrel. This last problem certainly affects the prototype gun, as can be seen in this picture from Mr. Crumling's website. The spray of sparks is grains of powder that were blown out of the cartridge and burned in the air; they didn't contribute anything to the energy of the bullet. Now, Mr. Crumling says that he's still working on the "load", which is the combination of bullet weight, powder type and amount, and he may very well be able to reduce the amount of unburned powder, but that doesn't give the bullet more energy.
Third, manufacture of the .314 Atlas cases is a precision machining job. Mr. Crumling is obviously a skilled machinist and has been able to turn out quite a few, at relatively low materials cost but a considerable investment in his time. They could also be manufactured by a machine shop, though there is some question about what legal hoops would need to be jumped through for that to be acceptable. In any event, though, there are two possible choices for obtaining Atlas cases: become a machinist (if you aren't already one), or buy them. It's also true that those are the two choices for obtaining conventional gun parts, and have been for some time. So if you're going to need to invest skills or money, why not just make or buy a regular pistol barrel and design a 3D printed gun around it? It might not be as cool, but it would likely be a much better gun. Or you could make a crude gun with little or no machining skill; something like a Colt Liberator or the "zip guns" that have been occasionally manufactured even in prison.
Fourth, the barrel portion of the Atlas case isn't rifled.
The drawings show a couple of gun frame designs that include printed
barrel sections with rifling, but it's obvious that they won't have any
effect on the bullet. If they are sized to actually engage the bullet, they'll be broken off by the first shot; if they're larger,
the raised rifling won't do anything and will be worn away by the hot
gas in just a couple firings. It might be possible to machine rifling
into the case, though that would be very difficult; I'm not confident
that it could be done at all in the current one-piece design. Perhaps if the case were threaded so that the "barrel" portion could be separated from the base, but then the design would be much more complex. An unrifled gun with a one-inch barrel is going to be inaccurate at anything but extremely short distances, a matter of a few feet; even for close-range use like home defense I would not want to trust its accuracy.
Finally, the lack of rifling may be the downfall of this entire concept. I can't see how anyone would make the case that the printed barrel has any function, and therefore the only barrel that this gun has is the smooth interior of the case. Entirely apart from the issues with accuracy and power, that appears to create a serious legal problem. There are a whole class of guns with smooth barrels, called shotguns; they generally fire a cluster of tiny round pellets, or a single bullet that has rifling grooves molded into its exterior to provide some spin. They're perfectly legal and used by thousands of sport shooters and hunters. But the law says that they have to have long barrels. Pistols with smooth barrels aren't part of what the law considers "firearms"; instead, they're in a category called "any other weapon" or AOW. They can be legally manufactured in many states, but must be registered and a fee of $200 paid for each gun. If that isn't done, the owner is liable for severe penalties. And in states with restrictive gun laws, AOWs aren't legal at all. Whether a gun based on the .314 Atlas is an ordinary pistol or AOW is certainly not for me to decide, but the question is undeniable and the chance of running afoul of the law isn't worth the risk, to me.
What's the conclusion of all this? I think that the .314 Atlas is an interesting and innovative idea, though not especially practical. It might become a stronger influence if it causes a realization that a fully 3D printed gun is a dead end, and that it makes more sense to incorporate 3D printing where it has a strong role, for example in producing customized grips and accessories, rather than trying to make it serve in roles for which it isn't suited. Perhaps there will also be a new direction of producing metal parts that are specifically designed to be used with 3D printed components; there could certainly be interesting designs made that way. But this isn't the dramatic development that the popular press would have us think.