The only part that is illegal to make on your own is the part with a serial number on it, for tracking purposes.
Perhaps that is the case elsewhere, but to point out that in the US, it is legal on the Federal level to make your own complete firearm for personal use. Assuming no state specific laws prohibit it, it is by default legal. ATF FAQ page. If you are not an FFL holder, and are not going to sell the firearm it does not need any serial number. All NFA restrictions still apply to homemade firearms.
The practice of legally homemaking firearms pre-dates 3D printing, with 80% AR-15 lowers being a modern and widespread example.
Perhaps that is the case elsewhere, but to point out that in the US, it is legal on the Federal level to make your own complete firearm for personal use. Assuming no state specific laws prohibit it, it is by default legal. ATF FAQ page. If you are not an FFL holder, and are not going to sell the firearm it does not need any serial number. All NFA restrictions still apply to homemade firearms.
The practice of legally homemaking firearms pre-dates 3D printing, with 80% AR-15 lowers being a modern and widespread example.