I'd never heard of it before you brought it up...so I looked it up.
Here is the basic information, grabbed from the interweb. Interesting.
"The M5 is actually based on the underrated, now obsolete High Standard Manufacturing Flite-King pump-action shotguns. High Standard used to market its shotguns as the “world’s fastest firing pump-action shotguns.” They eject by gravity alone. Sounds crazy, but flip the M5 upside down, click the slide release and it does just that."