Shereka Marsh via Facebook |
Shereka Fab-Ann Marsh died after a single bullet from a counterfeit 1930s Italian Beretta pistol struck her in Hackney, east London, in March.
The boy, 15, was cleared of her murder but found guilty of manslaughter.
He was also found guilty of possession of a firearm with intent to endanger life, at the Old Bailey.
The boy, who cannot be named for legal reasons, was remanded in custody for sentencing on a date to be fixed.
In March, Shereka had gone to give the boy a birthday present, the court heard.
After the shooting the boy called 999 but Shereka had suffered serious internal bleeding and could not be saved, jurors heard.
In court, the boy recreated the moment the incident happened, holding his hands together and pointing his fingers towards jurors from the witness box.
Under cross examination, he showed how he was holding the gun with both hands and "waving it" around when he "accidentally pulled the trigger".
He denied that he was posing with the weapon and that was why he had to have his fingers on the trigger.
These cases of possessing guns and playing with them is playing over and over again. This is why I always ask people to REPORT illegal guns! Get them off the streets and homes to avoid deaths like this.
#RIPShereka
With Files from the BBC