Guns are made by individuals all the time. They first came into use in the 15th century.
Buy you know that.
Lots of people build guns. Simple ones are easy.
We are dealing with people who can’t change a light bulb.
When I was younger a criminal built his own gun and used it to murder officer Dave Chetcuti of the Millbrae PD.
My thought is that if he is able to build the device, his brain functions enough to execute him.