AFAIK, The NATO Type AR shoots the smallest bullet ever used by any military anywhere, from the smallest & lightest rifle ever used by military anywhere. (
But if you can't hit somebody twice with one you may need some more practice. )
It's a totally modern weapon but we don't use it because it's the most
deadly weapon you can get. In fact it is far from it.
We use this weapon because it's easier on our soldiers, and therefore they do a better job with it.
It's a much easier gun for people of ordinary strength to use. The gun and its ammunition are much easier for them to carry.
Anyhow, for some idiot to claim that semi-automatic rifles
are too dangerous for ordinary citizens, the statistics prove just the opposite.
The thing that's dangerous is that we don't have enough security in many places to dissuade Mass murderers from their goal. They are able to just walk right in and shoot up the place because there's no one there to shoot back.
Now this is not the land of the police state where we need armed guards on every door. Like ancient Athens our security is in the willingness of the Common Man to take up arms in defense.
But instead of teaching ordinary citizens to defend themselves we teach them to run and hide, while they wait for their bullet.