Mohammed Guetfi, 33, of Green Lanes, was described in court as the “right-hand man” of prolific gun and cannabis dealer Eric Aram.
Both men were jailed for possessing firearms and supplying cannabis at Isleworth Crown Court.
Police began investigating the duo in April 2023 after officers found a converted blank-firing handgun stashed in a plastic bag at a flat in Bayswater.
Aram’s fingerprints were found on the pistol and his home in Hayes was subsequently raided in November that year.
There, officers recovered a manual and container for the blank-firing pistol, as well as a drill believed to have been used to convert firearms.
Cash worth £1,000 was also found in a vent in his bedroom.
Police say that incriminating text messages were found on a phone belonging to Aram, which revealed that the 35-year-old was a prolific gun and cannabis dealer, who converted and sold multiple firearms on the black market.
Gutefi was identified from his mobile phone downloads, and was arrested in January 2024.
The 33-year-old has been jailed for 14 years and four months, with Aram sentenced to 21 years and seven months in prison.
Detective Inspector Sam Bennett, who led the investigation, said: “On average, firearms are linked to over a thousand offences on London’s streets each year.
“Many of the guns used are reactivated pistols, converted by criminal gangs and dealers.
“Investigators used extensive digital evidence to bring these two offenders to justice.
“This meant they were able to secure convictions against both men, despite the fact that no guns were found on the defendants themselves.
“These convictions send out a clear message that the illegal gun trade will not be tolerated in our city.”
