In 1996, Michael Harris and James Taylor, while working for Colt Manufacturing, designed and tested the Miniature Asssault Rifle System (MARS). MARS fired a specially designed 5.56mm x 30mm cartridge. The cartridge had a 55-grain projectile.
Colt filed a patent application in 1997 which was granted in 1998 as patent US5827992A. The patent was expected to expire June 19, 2016.
The Colt MARS weapon never entered production.
For additional information, see Christopher Bartocci of Small Arms Solutions article from July 13, 2012 published at http://www.smallarmsreview.com