A father and son supplied criminals with hundreds of deactivated guns and "kits" to convert them into live weapons.
William and Mitchell Greenwood denied selling the gun "kits"
William Mitchell Greenwood, 76, and Mitchell Verne Greenwood, 42, from South Wingfield, Derbyshire, sold hundreds of guns from their shop.
Convicted criminals told police how they had been sold the weapons and given advice on reactivating them.
A jury at Derby Crown Court on Monday found both men guilty.
The court heard father and son ran an antiques shop in Little Eaton.
There they sold legal weapons deactivated under Home Office legislation, but offered expert advice on how to convert the items into working firearms.
The court was told that during an undercover operation between 1998 and 1999, the Greenwoods offered a detective an array of sub-machine guns and pistols.
They had once joked that if he bought 10 French-made MAB handguns he would be given one free.
Jurors heard the Greenwoods also took out advertisements in gun magazines, detailing their stocks of weapons and conversion kits.
These include a barrel blank, a length of metal tube used as a gun barrel, and a reamer, a tool used to create a chamber inside the barrel.
The two men claimed they legally sold all the items and had never offered customers advice on the reactivation of the guns.
Both men denied a charge of conspiracy to convert deactivated firearms into live weapons.
At the end of the six-week trial detectives revealed that weapons bought from the pair had been recovered at the scene of eight murders, countless gangland shootings and a loyalist arms cache.
Police estimate as many as 3,000 weapons sold by the family were likely to still be in the hands of criminals.
Judge Alison Hampton adjourned sentencing for the preparation of medical reports
and granted both men conditional bail to appear at Nottingham Crown Court on
She said: "A custodial sentence is almost inevitable, you must be
prepared for a custodial sentence and must accept the same as others who have
received substantial custodial sentences."
Detective Inspector Dick Severns, who led the £1m, 11-year-inquiry, said
his investigations had exposed the Greenwoods as the quartermasters to the
Following the verdict, he said: "From inquiries resulting out of this
investigation we have also secured the arrests of 40 other people and recovered
a total of 420 firearms.
"The Greenwoods are people who stuck two fingers up at the law and were
looking for a loophole to try and supply firearms."
William and Mitchell Greenwood refused to comment on
their conviction as they left the court building with solicitors.