Dealer Paul Edmunds, 66, was found guilty at Birmingham Crown Court of supplying weapons and handcrafted bullets to criminals.
He was first arrested at his home in 2015, where he had armouries and made bullets to fit antique weapons which were then sold on.
The court heard how weapons supplied by him have been found at more than 100 crime scenes including gangland shootings.
The registered gun dealer is reported to have said in a police interview that he "didn't give a s---" who he sold to.
Edmunds made bullets for use in vintage weapons such as Smith & Wesson pistols from the US and 19th-century French and Russian guns. He also imported prohibited 1950s Colt pistols from the US.
Edmunds pleaded guilty to exporting ammunition and has been remanded in custody until December 20 for sentencing.
Police concerns that antique weapons are being used in crime has grown in recent years. Last month West Midlands Police DCI Rich Agar said: "The problem of antique weapons is another emerging threat we’re addressing – there could potentially be thousands of Second World War guns unaccounted for. Criminals can reactivate these guns and an ‘underground’ armourer provide bespoke ammunition.”
The antique firearms trade is braced for the outcome of a new consultation on the legal definition of ‘antique firearm’. The Home Office last month launched a fresh consultation on the topic. The legal definition is being reviewed as part of the provisions for the new Policing and Crime Act 2017.