Just a firearm related point of information

[Big_Mike]

11. Also, from a very technical infantry perspective

Most people in the attack fire at about rate of fire, and therefore run out of ammo at approximately the same time and need to reload. The designated automatic rifleman takes up firing during this period, maintaining fire downrange while his teammates reload. He then drops off his fire as they return, reloads in his turn, and repeats as necessary. This scenario is in open field movement to contact or deliberate attack. In built up areas (towns and cities), techniques differ. Someone else who trained for that can step in if needed.

You are in error regarding the attack/defend rationales. In the attack, you carry only so much ammo, so limiting auto fire is required. Also, you need to have some left at the end to defend against counter-attack. On defense, again aimed fire as so much auto fire is wasted from a rifle. Machineguns are entirely different weapon systems, and are designed for that type of fire. Shoot what you got, was always the call, saving some to beat off counter attacks.

The one case where everyone auto-fires, at least at the beginning, is in some ambush scenarios. Fully automatic fire is psychologically dominating, so a MG just going and going helps defeat the enemy on two fronts, not just one.

This is all at a military level. What you do in a zombie apocalypse is up to you and your favorite brand of zombie.