Ian from Forgotten Weapons had a great video on improvised firearms with the National Firearms Centre collection at the Royal Armouries, including 3-D printed firearms. The collection is used for historical reference, international terrorism deterrence, and police work. The firearms greatly range in complexity and expense.

https://www.youtube.com/watch?v=0TMrunbZLJw

Here's another one on 1920/30s Chinese 'mystery' guns:

https://www.youtube.com/watch?v=4HNaB7l2GQk

His videos on Khyber Pass firearms are also fascinating.

Ian's channel is some amazing stuff for anyone with passing interest in firearms, and it's largely free of politics. Well, as much as it can be.