An international investigation led to the dismantlement of 10 illegal firearms manufacturing sites in Canada, Germany, Spain, and the UK, Europol said on Tuesday.
As part of the investigation, authorities arrested six suspects in Spain, one in Sweden, and three in the UK.
During the operations, law enforcers seized 70 illegal firearms, including assault rifles, 3D-printed semi-automatic submachine guns, pistols, and drill presses.
They also seized other weapons, including tasers, compressed air/blank firing weapons, crossbows, bayonets, and butterfly knives.
Other illegal material seized included over 37,000 rounds of ammunition, explosives precursors, gunpowder, and a grenade launcher as well as weapon parts and components, such as barrels and silencers.
The Spanish Civil Guard launched the investigation into the manufacture of illegal firearms earlier this year.
"Clues pointed investigators to individuals across Europe and North America who had purchased specific tools to manufacture or modify firearms," Europol said.
It added that some of the individuals arrested had previous criminal records, including for firearms-related crimes.
In some cases, the individuals used heavy industrial machinery, such as lathes and milling machines, to manufacture the illegal weapons, Europol also said.
In other cases, suspects used 3D printing to produce some of the components needed to assemble the firearms.
