People always rant and rave about AR-15s and, of course, they get all kinds of stuff wrong, like the civilian who insisted that my PT uniform wasn't a uniform, it was an outfit.
Civilians generally don't get to decide how the military does things.
When I was in the Army our M-4s and M-16s constantly malfunctioned. Whenever something goes wrong for me people always say "Obviously you were doing something wrong!"
All of our rifles malfunctioned, we cleaned them after every time we went to the range, and our Drill Sergeants inspected them.
I read a long history about the history of the AR-15 in the Atlantic and it explained that Eugene Stoner designed the weapon to use stick gun powder and an Air Force General adopted it in 1962, but the Army fought it for years, and when they finally agreed to use it, they insisted on changing it, and each of those changes made things worse.
They rigged tests to make the M-14 look better--and threw out data that made it look worse.
I read elsewhere that they altered the rifles they tested in Alaska so they failed that test. Stoner immediately flew up, inspected the weapons, undid the alterations, and they worked perfectly, but the Army said they needed to tighten the rifling to make the rifle work better in extreme cold, despite actively fighting in Vietnam.
They also insisted on adding a forward assist, which the Air Force fought, as well as Stoner.
The article says the Marines fought it, too, although I read elsewhere they didn't care.
One source said that troops found it to be helpful, but the Air Force ordered tens of thousands of rifles without it.
Also, the Atlantic said that the Army came up with specifications out of nowhere and would you believe that the gunpowder that Stoner specifically chose for his weapon, around which he designed his rifle, didn't meet the arbitrary standards, but gunpowder made by the manufacturer the Army used for decades did!
So, the M-16 rounds traveled just a little faster than the AR-15, but it also cycled much faster, got carboned up, and I believe there were other problems.
At first the Army said "No, that's too fast," but eventually said "This is fine," but cycling too fast caused all kinds of problems.
After Vietnam they modified the M-16 and the ammunition to work better together, but still not nearly as well as the stick gunpowder for which the AR-15 was designed.
I read elsewhere that the Army determined that a round needed to travel at a specific velocity in order to penetrate a US Army helmet and that was the velocity that the Army adopted, but velocity is only half of the momentum equation.
While I was deployed I posted a picture of myself in armor and a friend commented "You know our ACHes don't do any good, right? We shot one in the range and the bullet went right through it!
"
"You know that my mom will see this, right?!"