Here's a very interesting test of a rack-grade M16 by an AMU expert, with his analysis of how and why it performed differently from his match rifle:
https://youtu.be/0tv61Uqh16M (https://youtu.be/0tv61Uqh16M)
A very informative interview. I'd never been aware that three-round burst triggers have different pull weights at the various positions along the cam.