Paul Edmunds, 66, imported a Colt pistol used in a fatal shooting at a London nightclub and supplied ammunition used in two other killings and an attempt to shoot down a police helicopter.
A jury at Birmingham Crown Court heard Edmunds, of Bristol Road, Hardwicke, Gloucestershire, was arrested at his home in 2015, where he had three armouries and made bullets to fit antique weapons.
The registered gun dealer, who said in an interview he "didn't give a shit" who he sold to, was found guilty of conspiracy to supply firearms and ammunition and smuggling banned Colt handguns into the UK from the United States.
Jurors also convicted the pensioner of possessing a prohibited air pistol and perverting the course of justice by filing down a bullet-making tool to destroy potential evidence.
A two-month re-trial was told Edmunds and middleman Dr Mohinder Surdhar - likened by police to the lead characters in the TV series Breaking Bad - acted together to supply antique revolvers and custom-made ammunition to criminal gangs.
For his part, Surdhar, 56, from Grove Lane, Handsworth, Birmingham, admitted conspiracy to supply firearms and ammunition between 2009 and 2015 before Edmunds' trial.
Edmunds had denied hand-crafting bullets for use in supposedly obsolete vintage weapons, including 19th century revolvers, which he brought into the country legally.
As well as importing antique weapons, the gun fanatic also imported hundreds more modern prohibited firearms from the United States, having falsified customs paperwork.
Opening the case, prosecutor Andrew Fisher QC alleged "tell-tale" marks on ammunition found at crime scenes linked the rounds to tools used by Edmunds.
Edmunds, a former photographer, denied any wrong-doing by importing antique firearms into the UK legally, claiming he had never made and supplied live cartridges for the weapons.
But the court heard how Edmunds supplied guns and live bullets to criminals through Dr Surdhar, who he met at a gun fair.
The jury were told Edmunds' bullets were recovered following the Birmingham murders of Derek Myers in 2015 and 18-year-old Kenichi Phillips in 2016.
Detectives discovered that one weapon - imported on November 14 2013 - was used five weeks later in a Boxing Day fatal shooting at the Avalon nightclub in London.
Four of Edmunds' bullets were recovered from the victim's body.
Following the pensioner's arrest, 100,000 live rounds were seized from the armoury inside his garage, while seven wheelie bin-loads of gun and bullet components were recovered from a bedroom and attic.
In all, 17 criminally-linked weapons recovered by police are known to have been imported by Edmunds, while around 1,000 bullets connected to him have been recovered from crime scenes in the West Midlands, London, Greater Manchester, Nottinghamshire, South Yorkshire, Leicestershire, Warwickshire, West Yorkshire and Derbyshire.
The crime scenes included around 50 gang-related shootings and premises raided by police between 2010 and 2016.
But police believe many other guns seized by police across the country were sold by Edmunds, who failed to keep accurate records in his firearms register.
He was snared after experts at the National Ballistics Intelligence Service carried out an analysis, uncovering the growing trend of pre-war pistols used in a rash of shootings.
After the trial, Detective Constable Phil Rodgers, who led the investigation, likened the pair to the characters Walter White and Jesse Pinkman from the hit TV show Breaking Bad.
The officer said: "They were like the Breaking Bad of the gun world - on the face of it, both decent men, but using their skills and expertise to provide deadly firearms.
"But this was no TV drama - these were real weapons; real bullets; real victims. Their actions have had a devastating impact on communities by fuelling violent crime, leading to fear and bloodshed."
Edmunds' barrister acknowledged that the gun-dealer faces a sentence of at least 25 years when he returns to court, next month.
After being asked to consider a sentence giving the pensioner "some hope" for the future given his age, Judge Richard Bond cited the case of a 101-year-old man he jailed for 13 years in December last year.