NATO Designation

What is the purpose of the “NATO” designation in regards to ammo?

In regards to what specifically? Ammo?

Ammo yes.

Supposed to indicate interchangeable ammo between NATO countries, but I can recall this stuff hitting the market with the slower chronograph results and most people reporting malfunctions.

https://rifleshooter.com/2015/01/british-ss109-5-56mm-ammunition-cartridge-caliber-5-56mm-ball-l2a2-united-kingdom/

Maybe this will answer your question.

https://en.m.wikipedia.org/wiki/5.56×45mm_NATO