The Bullets (or pistol balls) are Inventory Items that can be used in the game.
The Bullets are large round metal balls similar to musket balls.
The Bullets can be found on a ceramic plate on the table in the Guardroom.
When Josef is under the table in the Guardroom he can push the leg of the chair when the Thin Black Cap guard leans back in the chair. The guard will fall back on to the floor giving Josef the chance to grab the Bullets off the ceramic plate.
The Bullets are used in the prison escape mission.
Josef will first have to steal the Prison Cells Key from the belt of the Thin Black Cap. (See: Prison Cells Key)
When Josef has the Prison Cells Key and the Bullets he will have to place the Bullets on the floor of the Guardroom by the prison cells security panel (the panel with two red lights). He will then have to use the key on the two locks on the security panel. This will unlock the cells and the Prison Robots will try to sneak past the guard. However the guard will notice that the prisoners are escaping and try to catch them. Luckily the guard will slip on the Bullets and the prisoners will be able to escape from the prison. The Thin Black Cap guard will then run out of the prison after the escapees.