Detectives said Paul Alexander, 53, was a "significant" supplier and had been tracked down following the launch of an operation against gangs in the wake of the shooting of 11-year-old Rhys Jones in Liverpool two years ago.
Police said they found a "gun factory" at a house Alexander rented in Bardfield Saling, near Braintree, Essex - and his weapons and ammunition had been linked to 28 firearms crimes in the UK.
Investigators said Alexander used 30 aliases and ferried guns and ammunition to the Midlands and North West in hire cars.
Chelmsford Crown Court was told today that Alexander had admitted possessing firearms and ammunition with intent to endanger life, converting imitation firearms into real firearms, buying and selling prohibited ammunition and money laundering.
He is due to be sentenced in November.
Police said Alexander was born Paul Daintry in Bury, Lancashire and had lived in Stoke-on-Trent and Bath as well as regularly travelling abroad.
Detectives suspect he made hundreds of thousands of pounds and an investigation aimed at locating and seizing assets is under way.
Alexander was arrested in September 2008 after a 12-month operation involving detectives from Merseyside, Essex and investigators from the Serious Organised Crime Agency (Soca).
"The guns and ammunition supplied by Paul Alexander are being linked to numerous shootings," said Mick Layton, deputy director of Soca.
"Put simply, he was dealing in death."
Investigators said they hoped Alexander's arrest had dealt a significant blow to gangs operating in the Midlands and North West.
Detectives said Alexander had served in the Royal Artillery and left in 1992 with an exemplary record.
He had come to the attention of police in 1995 when he was accused of arson after setting fire to a council flat he lived at in Stoke-on-Trent.
Lawyers for Alexander told Chelmsford Crown Court that he had complained about being regularly burgled but could not make a local authority listen.
They said he had started the fire as a protest.
Police said Alexander had been charged but failed to appear in court.
He had then been arrested in December 1996 by members of the United States Border Patrol when trying to travel into the United States from Canada on a false passport.
Alexander had been convicted of a passport offence and spent several months in prison in Vermont before being deported back to the UK and given a 12-month prison sentence for the Stoke-on-Trent arson.
In October 2002 he had been accused of causing an explosion in the garden of a house in Stone, Staffordshire.
His lawyers told Chelmsford Crown Court that he had been undertaking "surveillance work" for a client and had ignited a string of fireworks in order to distract security cameras.
A detective said he had again gone missing and been caught in September 2003 in New York, having travelled from Brazil on a false passport.
Alexander had once more been returned to the UK and had been jailed for explosives offences at Stafford Crown Court in 2004.
He had been released in 2005 and set up home in Bath.
Police said that despite Alexander's criminal background detectives had been unaware of his gun-dealing activities until he was arrested at Bardfield Saling in 2008.
Police said after leaving the army Alexander had purported to be an author - detectives said he had published two "vanity" thrillers which had not sold - and to be involved in "security" work.
And they said he had used around 30 aliases. One of them was Franz Bauer - thought to be taken from Jack Bauer, a fictional government agent in the television series "24".
Another was John Bourne - thought to have been taken from the fictional government agent Jason Bourne, who features in the novel and film The Bourne Identity.
Police said Alexander had been found after detectives in Merseyside launched an operation aimed at two feuding gangs in Liverpool, The Croxteth Crew and The Strand Gang - following the killing of Rhys Jones in 2007.
Police said officers had recovered a number of firearms from an address in Merseyside and found Alexander's DNA profile on a self-loading pistol.
In June 2008 his DNA was found on two similar weapons seized during an investigation by Greater Manchester Police.
SOCA had then launched an operation aimed at finding Alexander and he had been arrested in Bardfield Saling in September 2008.
Police said Alexander had created a "gun factory" in an outbuilding of the house, which he rented for £3,800 a month.
Officers had discovered about 28 firearms, including handguns and rifles.
They said Alexander had used skills learned in the army to adapt and convert imitation and fake guns.
Police said there was evidence that he bought imitation and antique guns - plus ammunition - from dealers within the UK and Germany.
Forensic experts found telltale signs on weapons Alexander worked on then found similar marks on guns seized following shootings.
Detectives said Alexander's wife Caroline Hunter-Mann-Purdy, 59, and his step daughter Rachael Hunter-Mann had also admitted being involved in money laundering.
Both were given two-year community orders by Judge Charles Gratwicke at Chelmsford Crown Court today.
Rhys was killed as he walked home from football training in Croxteth Park, Liverpool, in August 2007.
Gang member Sean Mercer, 18, fired three shots from a 1915 Smith & Wesson revolver across a pub car park at opposition gang members - the second bullet hit Rhys.
Mercer was given a 22-year jail term after being convicted of murder.
Detectives said there was no evidence to suggest the gun used in the shooting had been supplied by Alexander.
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