The M16
The M16, firing 5.56mm NATO, is in its sixth decade of service with the US Army: that makes it the longest-serving rifle in modern military history without a re-chambering. Sure, a modern M4 might not look much like a Vietnam-Era M16A1, but the same operating system and bullet have been around, and in effective use, for long enough that three full generations have put it to use. It’s pretty clear, then, that the Army is attached to their M16, and for good reason: it has proven to be a light and reliable weapons system in a wide variety of conflicts.