Jack Moran, this happens constantly, I don't blame you.
A bullet casing is a jacket, it surrounds a projectile core, a bullet can shed its jacket but it's usually down range. The police probably found shell casings for that SKS, and not bullet casings. Those shell casings would have come from whole rounds which once held a projectile or bullet, the propellant to dislodge or fire it from the casing, and a primer held in the shell base used to ignite the propellant when the primer is struck by the firing pin. The spent primer remains with the shell.
A bullet is a component of a round or shell. Victims are penetrated by fired bullets, the casings are left behind as witnesses. When you want to say or write bullet you most probably mean shell or cartridge or round. But good on you for using magazine over clip.