Top Definition
1. (n.) A weapon (generally an assault rifle) chambered in 5.56 NATO caliber.
2. (adj.) of or in 5.56 NATO caliber.
1. "I count three hostiles: two AK's and a five-five-sixer, over."
2. The new five-five-sixer HE round was especially effective against lightly-armored vehicles and personnel.
by Kirk0007 July 26, 2010

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