In the US this is a popular (and legal) pursuit though quite properly it is an offence to misuse any firearm. In Australia it is definitely a criminal offence to make your own, even a shanghai (see below). Mind you the fact that here (despite our draconian firearms laws) the great majority of guns ever sold to us remain unregistered and their owners unknown indicates to me that there is a great interest in this topic, and doubtless many thousands of practicing enthusiasts.
I knew of a chap who custom made 12 gauge shotgun-based shark killing guns for example. He supplied a ready market. I have known quite a number of other people over the years who have manufactured their own guns. Some even went on to patent them and attempt to sell them on to the armed forces!
The famous gun buybacks and gun registration laws only account for about 20% of the total number of firearms definitely known to exist in Australia: https://www.foaa.com.au/buy-back-statistics-and-australia-stock-of-firearms-compiled-in-1998/ In other words at least 80% are unaccounted for.
Hitler’s Germany introduced a ban on privately owned firearms which carried the death penalty. Though the ban still exists but does not carry such a hefty penalty, after three generations a very small percentage of the guns which were the target of the ban have ever emerged – which goes to show that the public’s belief in private gun ownership is very strong.
There are so many desirable designs: I particularly like this guy’s DIY air machine gun: http://thehomegunsmith.com/ This site introduces you to the DIY AR-15: https://survivalblog.com/building-your-own-no-ffl-ar-from-an-80-complete-receiver-by-jag/
There are lots of sites eg https://www.pewpewtactical.com/diy-gunsmithing/ & https://www.gungods.net/5-beginner-diy-gunsmithing-projects/ & http://www.shootingtimes.com/gunsmithing/10-must-have-tools-for-the-diy-gunsmith/ . A simple Google search for ‘DIY gun’ or ‘firearm’ will be most instructive.
It may now be illegal even to publish (or own) such instructions in Australia. We are rapidly descending into tyranny – and perhaps in an attempt to protect ourselves from a worse tyranny.
Picture yourself (with Churchill) and one of these:
PS: A helpful and thoughtful comment from Scott: ‘Checkout Clinton Westwood on Youtube https://youtu.be/WfvJtjbY9TM He has several interesting builds including rifling your own barrels. Our ridiculous laws only stop the law-abiding from making them, such a shame as it would be such an interesting project’.
PS: DIY firearms success in the US: The guy who invented the printable gun has had a huge win in court. Interesting times: https://www.wired.com/story/a-landmark-legal-shift-opens-pandoras-box-for-diy-guns/
Their sire https://defcad.com has once more been (temporarily) shut down by legal action.