The M16 story begins in the early 1960s, when the U.S. military sought a lighter, faster rifle than the 7.62 mm M14. Eugene Stoner’s small-caliber AR-15, with its aluminum receivers, steel bolt and barrel, and plastic furniture, became the basis. In 1964 the XM16E1 entered service as the M16, and in 1965 it went to Vietnam. Its lightweight 5.56×45 mm cartridge and straight-line recoil layout promised controllable automatic fire and easy carry for long jungle patrols. Combat exposed hard lessons. Early rifles paired a three-prong flash hider and 1:14 rifling with unchromed bores, and they met humid mud and corrosive climates head-on. Jams and fouling fed a reputation problem. The response was iterative: chromed chambers and bores, a 1:12 twist for the 55-grain M193 and M196 tracer, a closed 'birdcage' flash hider, protective ribs around the magazine release, and a forward assist to help seat a stubborn bolt. In 1969 the improved M16A1 replaced the M14 as the standard service rifle. Under the skin the rifle ran Stoner’s expanding-gas system: propellant tapped at the barrel drove a lightweight carrier and rotating bolt without heavy operating rods. With the recoil spring in the stock and the bore in line with the shoulder, muzzle rise stayed low and follow-up shots were quick. A 20-round magazine grew into a curved 30-rounder, and the disposable aluminum mag became a NATO standard, later refined with anti-tilt followers and stronger springs. By the 1980s the rifle matured again. The Marine Corps adopted the M16A2 in 1983 and the Army in 1986, switching to the heavier 62-grain SS109/M855 round and a faster 1:7 twist. A thicker forward barrel, new handguards, case deflector, revised sights, and a semi-auto and three-round burst selector reflected a shift toward durability and controlled fire. Accuracy remained a calling card, helped by light recoil and a long sight radius. Optics and modularity defined the next step. Adopted in 1997, the M16A4 traded the fixed carry handle for a flat-top Picatinny rail and removable handle, inviting ACOGs, red dots, and backup irons. The same rail culture spread to under-barrel launchers like the M203 and M320, and to accessory shotguns. In the field, Marines often kept 20-inch barrels to wring velocity from M855, while the Army leaned harder into the compact M4 carbine. Ammunition shaped performance as much as steel and aluminum. The original M193 fragmented violently at high impact speeds but lost that behavior at distance; M855 added a steel penetrator for armor, trading some soft-tissue effect. Reports of inconsistent wounding from short barrels pushed development of the lead-free M855A1, tuned to fragment reliably and penetrate more while keeping accuracy from both M16s and M4s. Through decades of patrols and ranges, the M16’s ergonomics and accuracy kept it relevant, even as reliability lessons from Vietnam and later dust-chamber tests drove constant tweaks to gas ports, finishes, magazines, and maintenance. Total production climbed toward eight million worldwide, and many allies adopted local versions built around the same mags and manuals. By the 2000s the lighter, handier M4 largely displaced the M16 in U.S. frontline units. In April 2022 the Army selected the SIG MCX SPEAR, designated M7, to begin replacing the M16/M4 family. That marks a new chapter, but the M16’s arc remains clear: a lightweight rifle born in the jungle, refined by hard use, made modular by rails and optics, and influential enough to define how modern forces think about recoil, accuracy, and the utility of a standard 5.56 mm service arm.