From Wikipedia:

Despite its early failures the M16 proved to be a revolutionary design and stands as the longest continuously serving rifle in US military history.[59] It has been adopted by many US allies and the 5.56×45 mm NATO cartridge has become not only the NATO standard but "the standard assault-rifle cartridge in much of the world."[60] It also led to the development of small-caliber high-velocity service rifles by every major army in the world.[61] It is a benchmark against which other assault rifles are judged.

It's amazing to me how many years of R&D it took to develop the M16. Of course that coincided with substantial improvements in manufacturing techniques between the end of WWII and the late 50's. Things like stamped parts and industrial grade plastics revolutionized not only weapons but consumer products as well.