What are NATO rounds and why are they banned in the US?

NATO rounds refer to the standardized bullet used by most/all NATO countries, such as the 5.56x45 and 7.62x51mm rifle bullets.

Neither of those are illegal. In fact, they are both very popular on the civilian market. There are some incredibly stupid politicians trying to make them illegal on the grounds that they are used by the military to kill people even though there are far more dangerous bullets out there.

That being said, there are better bullets out there for civilian application.

a 7.62x51mm round. It is called NATO because its a standard round in most NATO countries (most of those countries have weapons that use the 7.62x51mm round). The ammunition itself is NOT illegal in the US however almost every weapon that can fire that round is illegal.

They aren't banned in the US.