Simple it was adopted by the mayor world power.
I qualified as ‘expert’ marksman, 1967...js
It didn’t weigh much and lots of ammo for it could be carried without much effort. A perfect infantry platform.
It was a fine weapon, lightweight and easy to fire. I carried one every day for 4 years in the Air Force and even though I haven’t handled one in 40 years, I bet I could still strip it, clean it, put it back together, and do pretty decent at hitting my target.
What the USA should have built was a “miniature” M1 rifle that would accept a 20-round magazine & in a cartridge of the size/power of the .250 Savage.
(By 1947, BERETTA was converting M1 Rifles to 7.62x51mm, which is VERY similar to the “old-school” .300 Savage cartridge).
Yours, ex-cadet
The M16 sucks. On 2 occasions (firefights with the Viet Cong) it did not properly chamber the ammo properly and resulted in jams that put my life in danger.
It was the first step to arguably the most versatile individual rifle platform ever. The mil-spec standard is what created an industry that could support endless improvements for the user, including numerous cartridge/upper choices. Low recoil, huge choice of optics/sights, easy to maintain. Today’s versions are not the 60’s mattel guns.
If you look at the rifles that preceded it, you will see that when comparing the M-1 Carbine, the M-1 Garand and the M-14, that when we were in conflicts past WW2, the recoil of the M-1 Garand and the M-14 were considered too much for many soldiers to handle, especially,if fully automatic or burst fire was required. That is why the M1 Carbine was so popular especially in Vietnam with ARVIN troops of small build.
Likewise, in the era of the draft, the majority of the troops had little marksmanship experience. We were no longer a society of rural farm boys or muscular laborers and factory workers.
Why the M-16 became so popular was not the low capacity magazine or the heavy weight of the cartridges of the Garand or M-14, it was that fully auto or bust fire was the current military tactic and a 30-06 or .308 round (which required a heavy rifle) were to difficult for many of the soldiers of the day to accurately master.
That didn't mean that some could not attain a marksman's badge. It just meant that if you are standardizing on one rifle and ammo on a battle field for most of your troops including allies, you want something they can handle in a firefight.
You also need to understand that "Whiz Kid" Robert McNamara wanted one rifle to be used in all services for almost all soldiers. So the M-16 was to replace the M1/M2 carbine for something more deadly that could be given to truck drivers, clerks, etc. as well as serve as a main battle rifle.
Professional soldiers or those to trained like professional soldiers were a different story.
What a ridiculously crappy rifle.
Nothing.