I have touched on this a time or two before, but firearms are not exactly difficult to manufacture. Oh, sure, good firearms – y’know, the kind that are less likely to blow up in your face than not, that can hit a target reliably tens of yards away, etc. – require millions of dollars of fabrication equipment, testing, quality control and assurance, and the whole shebang of an assembly-line industry, but just a basic device that sends a pin into a primer and then controls the resulting explosion in a specific direction? Yeah, you really can kitbash something like that together in your garage.
Unfortunately, being something more of a theoretical engineer than an applied engineer (and being too lazy to go digging through the various resources on the topic), I never really knew exactly what was required to make that happen. Thankfully, Andrew S gave us the short break-down:
I’m a CNC machinist, and with $10,000 of equipment I could churn out 2-3 full auto rifles per week. They wouldn’t be pretty, or super reliable, or tack drivers; but they’d be good enough to spray-fire from the hip and do the work a criminal would need to do. With $50 worth of stuff from home depot, I could work up a pretty credible single-shot liberator style. A gun just focuses an explosion in one direction. Explosions are easy to make. I could even see a projectile weapon that uses an aerosol propellant as the explosion source. Or gasoline. Anybody with a couple of shop classes under their belt and a high-school chemistry education is fully capable of manufacturing lethal projectile weapons.
As I said in comments, I really wish my high school had a shop class… ‘Course, I probably would have gotten myself in a lot more trouble, but, hey…
But, regardless, CNC machines are not regulated. Drill presses are not controlled in any fashion. Engineering diagrams can disappear into the internet, never to be rooted out again, just as easily as celebrity sex tapes can. And making a firearm in the comfort of your own home is about as easy/hard – and about as Constitutionally-protected – as running your own printing business.
As I keep trying to tell the "gun control" extremists, the firearm Djinn is out of the bottle, and there is simply no way you are going to be able to shove it back in. Neither bans no registration/regulation will function to reduce overall crime or violence rates here in the States, not just because there is a negative correlation between firearm ownership and firearm-related crime rates, but because the firearm trade with and amongst criminals is already illegal and making it more illegal will not really change a thing in their minds. The black market will pick up, home fabrication of firearms will start popping up amongst the criminals who want firearms, and their world will just keep on spinning, but the law-abiding citizens who desire to procure and carry firearms for self-defense against that inescapable criminal element will be left forcibly disarmed and defenseless by the very laws supposedly intended to "protect" them.
You cannot "control" firearms any more than you can "control" any other idea or basic tool – sure, it may require a bit more effort to create than a knife, but now we are just talking a difference in difficulty, not outright possibility.