Here's a pic of the system and box that started the widespread usage of the term "SuperGun"
<img src="http://home.earthlink.net/~adrianmr/_uimages/supergun.jpg" alt=" - " />
My personal opinion as to why these devices were/are called SuperGuns is due to the fact that the creators are using a sort of play on words in respect to the electron guns in a monitor. A Supergun basically allows you to play arcade games, which are normally intended for display on RGB monitors, on a standard TV set. Therefore, the purpose of the Supergun is to do change the video output for use on a TV's screen's electron gun, thus, the term SUPERGUN was born.
I love the way these older Superguns look. I would love to track one down someday despite their less than stellar video quality and encoder type.