223 Remington), the same round used in the M16 and M249, and similar to the PLA intermediate round, the 5.8mm x 42mm. The M4A1 uses the standard 5.56mm x 45mm NATO intermediate rifle round (known in non-military rifles as the. Later, the M4 was adapted to fully-automatic fire instead of burst fire with the M4A1, and the original, non-removable carrying-handle iron sights were replaced with a rail system, like the one present on the M16A4 rifle, which allows the iron sights to be removed to mount alternate optics in a simpler and more ergonomic configuration than the older M4 and M16 rifles.
The M4 carbine rifle was originally developed by the United States Marine Corps as a shortened, three-round-burst variant of the M16A3 rifle.
