Re: What kind of 3D printing...
I apologize if my post failed to address the use of sintered plastic composites as an affordable and effective alternative to metal in a firearm barrel as manufactured by a '3D' printer using a standard metallic projectile. I shall attempt to do so below.
The effects noted in your provided example were achieved by the bullet destroying the 'rifling' and the bore of barrel as it traversed the barrel at an at an oblique angle to the bore. Basically scraping down the barrel and defining its own gas and pressure seal as well as trajectory as opposed to that dictated by the barrel. A gas seal and trajectory that cannot be intentionally repeated. You could achieve similar results by filling the bore with gelatin.The bore must remain, effectively, unaltered after each round is discharged in order to produce stable and safe results (as a gun that isn't reliable and safe to use is 100% worthless). Using sintered plastic composites is a guaranteed (exciting?) failure state becoming increasingly probable with each discharge. Below are the reasons why (I have attempted to keep the explanations rather simple, but not so simple (I hope) that they offend).
- Printable plastic composites cannot be made sufficiently rigid, at the scale required, to perform the act of grooving the bullet to maintain the seal between the breech and the end of the barrel. With a sufficiently large bore and projectile you could create a large enough bearing surface to prevent the instant and progressive degradation of the rifling but it would need to be far larger than an (easily) portable firearm. The lands in rifling (the part that stands proud of the inside of the bore) are quite sharply defined in order to effectively cut into the bullet to create and maintain the seal while minimizing removal of mass from the bullet as well as to minimize resistance in order to maximize exit velocity and reduce the likelihood of a round lodged in the barrel.
- Sintered plastic composites do no respond consistently or reliably to mechanical action in situations with rapid temperature fluctuations and/or high heat. The materials, in a sufficiently heated environment (as found in an automatic firearm), enter an inconsistently malleable state where they are deformed by mechanical action and again realign themselves as they cool based around the random positioning of voids present as an inherent part of the sintering process. Resulting in significant changes in shape throughout its length during and after each discharge. Initial manufacture under high pressure could reduce the random variations somewhat but that is far outside the scope of a 3D printer accessible to anyone who would need to print a firearm (as opposed to buying one or making one from traditional materials).
- The gaseous byproducts from the ignition of modern propellants are highly corrosive and contain a very high particulate content. Combined with the previously discussed nature of sintered plastic composites introduces a very high possibility of said particulates become trapped inside the cooling material and creating a highly unstable void that can cause catastrophic failure at the point of the particulate inclusion.
Those are some of the basic reasons why an affordable plastic firearm barrel is a poor idea.
Alternatives for 3D printing of finished goods:
Assuming you are not going to attempt to defeat a metal detector (which is a terrible plan from the start for a lot of reasons, but millimeter wave devices render that attempt completely unsuitable) but instead equip a group of disenfranchised or poorly funded rebels unable to buy or steal their own firearms.
A 3D printer could be used to create inexpensive, accurate and consistent molds for casting DIY weapons and even for the creation of primitive, but effective, barrel boring and rifling equipment. Sig Sauer has proven the casting process to be highly effective and cheaper than the traditional forging of firearms. A suitable furnace and could be constructed and low quality, but suitable, raw or recyclable materials could be built and acquired for less than $10k and could produce several reasonably reliable, reusable and effective firearms per day. The same process could be used to manufacture ammunition and ammunition loading equipment (less propellant and primer of course).
Such a use of a 3D printer is far more within its reasonable capacity and will provide a finished product superior in every aspect to that of a plastic composite alternative.