As federally-licensed firearms manufacturer (FFL07/SOT02), I can attest to how straightforward and even downright "easy" it is to create numerous firearms and firearm parts of varying legality. On the "not so illegal" side you have pistol slides, custom triggers, accessory railings, etc. that are all completely-legal to make, possess, sell, use, etc. It would be like 3D-printing a new case for a desktop computer.

On the other hand, you have things like removable drop-in auto-sears (RDIAS) [1], Glock selector switches [2], MP5 auto-sears, AR15 -> M16 conversion jigs, handmade silencers (suppressors) [3], and numerous other components that not only require a federal license to create/possess/use, but also require a special taxpayer status called an SOT (Special Occupational Taxpayer) [4] because all of the aforementioned items are construed as machine guns (even if the device merely converts a firearm to a machine gun, and of course silencers) under the National Firearms Act of 1934 [5], and later clarified with the Gun Control Act of 1968 [6].

In addition to those items, NFA also deals with short-barrel rifles (rifles with barrels under 16 inches in length) and short-barrel shotguns (shotguns with barrels under 18 inches in length) which are incredibly easy to "accidentally" own. However, the largest firearms loophole in the country is that by attaching a "pistol brace" to an otherwise regulated short-barrel rifle, you are in compliance. Hundreds of thousands of Americans are currently in violation of the National Firearms Act of 1934 because they possess these short-barrel AR-15's. The differences between these two configurations are strikingly and laughably small [7].

Especially with the machine gun parts, the idea of governments/municipalities expecting citizens to simply surrender them - even for "fair market" value - is simply absurd and altruistic.

Fun Fact: You yourself (in most states) can own a fully-automatic firearm! The only catch is that it has to be manufactured before May 1986, be registered under NFA, and transferred to you on an ATF Form 4 (which requires a $200 tax stamp [the same as it cost in 1934] and around 210+ days of waiting). These are known as "transferable machine guns" and are quite valuable despite their almost-identical resemblance to their semi-automatic counterparts. Here are some for sale: https://otbfirearms.com/nfa/transferable-machine-guns/

[1] https://www.recoilweb.com/turning-your-ar-15-into-an-m-16-15...

[2] https://buybestguns.com/product/semi-full-auto-glock-handgun...

[3] https://solventtrapsdirect.com/

[4] https://en.wikipedia.org/wiki/Special_Occupational_Taxpayers

[5] https://en.wikipedia.org/wiki/National_Firearms_Act

[6] https://en.wikipedia.org/wiki/Gun_Control_Act_of_1968

[7] https://www.guns.com/news/reviews/ar-pistol-vs-ar-rifle-what...