The M16 is .22 caliber but uses a cartridge called 5.56x45mm Nato. It is commonly referred to as 556, 5.56, 5.56 Nato, etc. Early AR-15 rifles were chambered in .223 Remington (very similar to the 5.56) and would continue to be on and off until the early 2000s when 5.56 became the more accepted chambering since it can fire both .223 Rem and 5.56 Nato with little issue.