I read this book back in the early 80s. It was a book of children's short stories based around Science Fiction. This story, The Master Blaster, was about a boy who is on holiday and looking for a toy gun, and walks into a futuristic shop (called WPNZ) where he buys The Master Blaster, believing it to be a toy gun, but instead, when he takes it to the beach and tries it out on a seagull, finds it's anything but.
The Master Blaster has three settings; it can make a person vanish, silent, or motionless...