M-1 (?m?w?n?)n. A .30-caliber, gas-operated semiautomatic rifle capable of firing eight rounds before reloading, used by US forces during the 1940s and 1950s. Also called Garand.[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.