What size ammunition does an M-4 use?

The Civilian and military version of the M-4 semi-auto rifle uses .22 (.224) caliber small centerfire rifle rounds. preferably 55 grain full metal jacketed rounds (most common). The grain refers to the weight of the projectile itself. One could fire a heavier grain projectile depending on what velocity and performance you prefer and attempting to match the bullet to the type of barrel on the rifle itself. Remington popularized calling the round simply a .223. NATO calls the round a 5.56.

5.56 is equvalant to 5.56 mm

.224 is 224/1000th of an inch