[Imp]

Its much more complex than that, training & what you are fighting for make a huge difference hence the reason we have experience & morale that varies by nation & time.
The amount of rounds fired always stagers me & its why armies went to semi auto rifles so suppression is more effective as that's what most shots are doing. Remember however these figures normally include the likes of MGs etc including vehicle & air.
Also the US army are a bit err liberal with their use of firepower.
On a side note Russia was not to worried about ATGMs till the stockpiles grew as they thought stocks would soon be depleted, many nations though still have very limited supplies.

Your Vietnam example is a good one & if I remember US army training was modified due to it.
In green units the figures were far worse more like only 3 or 4 men returned fire. This was also fairly common in WWII for a some nations.

When a unit becomes combat ineffective depends on many things, mainly training. But what you are fighting for also makes a big difference. Brain washed for a cause, defending home soil especially if your family is just behind your lines creates fanatics. Or as a major example whatever was instilled into the Japs in WWII. The US sending camera teams along with Marine landings etc is another example, Johnny's not going to hide in front of Ma & Pa.
Fighting to the last man & combat effectiveness are 2 different things however.
Kill the right man in a squad & it could become combat ineffective virtually straight away. Conversely if he survives its possible for him to instil acts of valour in a few remaining men.