Where did old Mona get the gun?

According to the TV series official website, Lao Mo's gun is a gun made by Gao Qichen, a sophomore in high school, and the homemade gun has always been in Lao Mo's hand.

The story told in this play spans twenty years. An Xin, played by Zhang Yi, used to be a high-spirited criminal policeman, and became a white-haired old policeman twenty years later.