US marine firing an M4 carbine fitted with ACOG 4x scope.
Source: US Marine Corps (LCpl Mark Stroud) - © Public domain
The M4 is the carbine variant of the M16A2. The M4 has a shorter barrel and retractable stock.
The Colt M4 is very similar to the older CAR-15, but is based on the NATO-caliber generation of the M16. Most variants of the M4 feature a heavier barrel than the CAR-15 and have a flattop receiver for mounting optics.
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