The gun is the author’s stylized version of an old rifle. Probably inspired by the ones you see in cowboy Westerns.
Her shirt is a puff-sleeved shirt.
I guess the hair would be called swirl pigtails, don’t know if there is an official Japanese name for them.
Not sure what I call this outfit, maybe school girl meets wild west. Stripped socks and corset seem very Saloon Girl (weird for a child’s outfit to be inspired by that). The boots are just cowboy boots.