M-16 (?m?s?k-st?n?)n. A 5.56 mm-caliber, gas-operated, clip-fed assault rifle, first used by US troops during the Vietnam War.[M, military classification.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.