She use magic, even if in a very kinetic way.
Is pointless to analyze why the legendary thousand folded katana cannot cut an entire building in half cause of the blade lenght.
She can use that weapon cause this is a manga and cause magic. Interesting how people can accept without any question that a traditional mage can move his hands a little maybe say some bullshit, and thanks to it he can literally dominate the laws of reality. But it become a problem when the gun is too big for the loli.