The Bartoli 12G is the default pistol used by guards in every mission. It can be acquired after killing/knocking out a guard or by finding it in certain areas like a security room. It chambers 12 9mm rounds and can refill the ammo of any other pistol in the game if picked up. It takes three shots to kill npcs if shot any where other than the head and one for the head (At close to medium ranges!)
In the prologue Freeform Training you can find the weapon in the little house right next to you on the left side, straight away after spawning. Inside the house you are also able to find the Fusil G2 assault rifle.
In The Showstopper mission, there is a Bartoli 12G lying on a table inside the security room, in the south east corner of the basement.