I've said it before and I'll say it again: The original AR-15 design is 58 years old (it's precursor design, the AR-10 is 60), the original AK design is 68 years old. The 1911, one of the definitive semi-auto pistol designs, dates from, you guessed it, 1911, with precursor designs and prototypes from the late 1890s. Machine guns predate the 20th century, and rapid fire crank weapons predate the American Civil War. Sniper rifles with effective ranges in excess of 800 yards were used to great effect in the aforementioned American Civil War, and current top-line sniper rifles are generally bolt action, which predates smokeless powder and metallic cartridges. Since all of these inventions, manufacturing technology has done nothing but improve. Basic machine tools, available to everyone, can be used to produce professional quality firearms. New developments, such as desktop CNC mills and 3-d printers simply reduce the amount of space and skill required.
So it should come as now surprise when criminals circumvent the law with homemade firearms.