It was not a sniper rifle.

It was a light machine shotgun.

It was a special gun based on the type of rapid-fire machine gun that could be carried around but was usually used while fixed on a tripod. It had been modified so that it only used shotgun ammunition. When fired at close range, the light machine shotgun held enough destructive power that it could transform an armored vehicle into what looked like a crushed can. It was Stephanie’s personal custom gun.

Toaru Majutsu no Index vol.19 ch.1 between the lines 1