an Ender 3's print quality is too low to reliably handle any of the critical components, even for one or two uses. something like the defcad AR lower receiver (which is for some odd reason designated as "the firearm" under ATF regulations...) can absolutely be printed, but not reliably by an ender 3- at least not a stock ender 3. (the defcad team was using resin printers for the dimensional accuracy.)

in any case, you can go to any big box hardware store, drop around 30 bucks in plumbing parts and some quality time with a dremel will produce a fully automatic firearm. should we now regulate plumbing hardware?