In practical terms, one interested in doing harm has far easier ways than buying an 80% receiver. I can change that to a block of steel, aluminium, plastic, or even plywood, as a zero percent receiver and make an AR lower receiver that would function for at least a few rounds without tremendous difficulty. Just like the Atomic bomb at the mass level, once the secret is known how to do it, the rest is just engineering a solution that is possible to make with what you can get. By and large the greatest supply of firearms for criminals are straw buyers and obtaining firearms that are stolen from law abiding owners.
For example as the Dayton and Fresno mass murderers used straw buyers to buy as did the Columbine killers. The CT creep killed his own mother as did the Oregon and MS school murderers who killed their parents in order to steal their firearms to carry out their dirty deeds. Stolen firearms plus straw buyers generate most of the illegal use of firearms in cases. A remainder is where the perpetrator of mass horror, like in LV, decides to go out in a horrific blaze of infamy and buys their firearm quite legally. Essentially they get revenge on the world for perceived slights and don't face consequences because they either commit suicide or make the cops do it for them.
The true fact is that any decent machinist in a machine shop can make a firearm that is functional and it is even easier to make a full automatic than a semi due to parts simplicity. A bolt action shotgun can be legally made with the right barrel length and so forth with parts that you find in the plumbing section of your local hardware store. Take a look at the Sten gun for example or the creative firearms made by those in Brazil or Australia. Thugs are gunna thug and mass murderers find it easy to substitute stuff for mass mayhem. Pressure cookers can be obtained by anyone and a resulting bomb can be triggered remotely by cell phones. It is a physical impossibility to ensure a "gun free world" as the benefits of making an illegal firearm can to some individuals outweigh the costs of incarceration. You cannot eliminate the knowledge nor the means of production without becoming a large prison camp. Note that in prison, despite the total lack of rights and the supposed inability of inmates to get contraband to kill one another, that inmates still manage to maim and kill even under the very noses of the authorities. The reason is that criminals commit crimes because that is what they do--the means by which they commit crimes may change but the desire by an individual to live the thug life is responsible, not the means.
The public know this more or less which is why the lack of support for a total ban on firearms in the U.S. The rest of the world is learning that laws do not magically make thugs obey them such as in UK, Canada, Australia, etc. and as NZ will soon learn. Criminals are going to be criminals wherever a lazy pool of violent and short sighted individuals exist that would rather be barbarians to loot and pillage than a peaceful contributing member of society. The best that you can do is incapacitate these individuals from harming innocent others by removing them from society via prison terms or worse. Some may eventually prove redeemable but obviously penitentiaries did not work the way that the Quakers thought they might otherwise we should be basking in low crime rates today resulting from mass incarceration.
