If you knew anything about guns, you'd know it only takes a few basic tools and materials to make a functional gun that goes bang without killing its user. You don't need a 3D printer. There's no way to disarm anybody in any circumstances.
I do know something about guns and about metal machining and fabrication work. Making a Sten is dead-simple. Heck, I've got the plans.
You're correct that between the staggering number of guns that already exist in the US (and the majority of rifles & shotguns never having been registered) combined with the ease with which a gun that's at least good enough to get an enemy's gun is to make conventionally, it seems pretty impractical in the short term.
However, there's "simple" for some people and then there's "simple" for everybody else. It's dead-simple *IF* you have a lathe, drill press, sheet metal brake, and maybe a mill depending, along with multiple other ancillary tools and pieces of equipment like an arbor press.
*AND* you *also* have the requisite training, skills, & experience to operate that fabricating equipment well enough to produce more than a modern-art piece or a way to assure that you never need worry if you lose one of your mittens and/or your sunglasses. It's not a trivial skill set in the least.
The difference here is that you basically only need the printer instead of a pole-barn full of expensive machine tools, plus you don't need any advanced machining & metal fabrication skills or training to fabricate high-quality components.
The printer/software and the plan file supplies the majority of the training, experience, and skills otherwise necessary, while replacing multiple expensive pieces of metal working & fabrication equipment while also requiring less space. More like residential garage/shed/basement-size instead of pole-barn size.
A metal printer would also be a much more practical solution in the city. The printer is also far more portable than a bunch of machine shop equipment. It can be relatively quickly moved between locations and concealed compared to normal tooling.
Wishful thinking... It's not the lack of guns that keeps your tyrannical government in place, it's the lack of courage in a population that has turned bovine, uneducated, and more interested in shopping and watching reality shows on TV than in fighting for liberty and moral principles.
I agree. However, I'm hopeful that people are beginning to wake the hell up. I haven't seen the current levels and breadth of dissatisfaction and anger with government since the '60s/'70s, nor anywhere near the current numbers of people who seriously think the government needs to spend less and have fewer powers, and are actively getting involved and doing something about it.
When was the last time you remember *this* happening?
There may yet still be hope. Especially if you consider it was only about 10% of the colonists at the time who were actively for the US Revolutionary War and independence from England.
Can we scrape up 10% with a brain and a spine these days? Who knows. We'll find out, I guess.
Maybe the concept of free men governing themselves by common agreement dies here forever, technology guaranteeing the jackboot continues forever grinding the human face underfoot.
Maybe humans need another few 10, 20, or 100s of thousands...maybe even millions...of years of evolutionary advancement before mankind is ready to leave kings, dictators, tyranny, and authoritarianism behind us.