What ammo does the colt m4 carbine use?

It fires the 5.56x45mm cartridge. It is compatible with the 62 grain M855/SS109 ball, the 64 grain M856 tracer, blank cartridges, and it can use the 77 grain Mk. 262 cartridge. The slow twist of rifling (1 in 7) makes it impractical for use with the older 55 grain M193 ball ammo and 45 - 55 grain commercial .223 Remington loads.