Can a colt ar-15 fire 5.56mm ammo?

Yes, a Colt AR-15 can fire 5.56mm ammunition, provided it is specifically chambered for it. Many AR-15 rifles are designed to handle both .223 Remington and 5.56x45mm NATO cartridges, as they have similar dimensions but different pressure specifications. Always check the markings on the barrel or consult the manufacturer to ensure compatibility before use.