Louis De Zoysa was convicted of murdering Matt Ratana after the shooting at a police station in Croydon, south London, in September 2020.
The 25-year-old had been stopped and arrested after police found ammunition and cannabis in his car, but they did not realise he was carrying an antique firearm in a holster under his armpit.
At the police station, De Zoysa managed to produce the gun despite being handcuffed behind his back, and fired four shots, also shooting himself in the incident.
He was charged two months subsequently, once he had recovered.
Ratana, a long-serving police officer, was fatally injured in the incident.
Met Police Commissioner Mark Rowley praised the bravery of Ratana's colleagues, saying that "without their courage I believe that more lives would have been lost."
"Officers never have a perfect picture of what awaits them at the next incident. Every day we take several firearms off the streets of London and the majority are seized by unarmed officers," he said.
The purchase of most guns in Britain is illegal and shootings are relatively rare in comparison to the United States.
De Zoysa had purchased the antique revolver legally under a "obsolete calibre" exemption, police said, and had made the ammunition at home as no ammo was readily available to buy for the 128-year-old weapon.
After Ratana's murder, the government introduced new laws to ban that kind of antique weapon.
