An M100 is a Semi-Automatic Carbine Assault Rifle made by the American gun company Calico. It is called the M100 because of its high capacity clip of 100 rounds + 1 in the chamber. Was very unpopular due to the assault weapon ban which has since been repealed and M100's are now legal for civilians to possess in their respective state. The military has yet to contract with Calico thus leaving Calico a relatively small gun manufacturer. In some states is also referred to as a "long rifle" making it legal to posess in states with an assault weapon ban.
Josh: "Did you hear all of those shots last night?"
Dave: "Yeah my boy brought his dad's Calico M100 to my house and we took potshots at that tree stump"
Josh: "Damn I musta heard at least 100 shots fired"
2. The Notorious BIG in the song Somebodies gotta die references it "What kinda shots my nigga I see mad holes. What kinda gats? Hitchlinks, cocks, and calicos"