I think sometimes many think they will be just as effective during a bad SHTF as during good times shooting in perfect weather from a benchrest at a paper target! Like you stated...big difference. I know what real hunger is & how it can effect performance...the same with bitter cold, sweltering heat, have-not-slept-for-days fatigue, fever, etc. Same goes for practicing under the above conditions...low light & night as well!
So I favor any edge I can get to compensate for less than favorable conditions....as well as against numeric disparity combined with the element of surprise. So....the most effective weapon for the majority of SHTF scenarios I can envision is the electro-optic dot sighted AR, AK or similar.... fairly lightweight, compact, reliable, low recoil, high capacity, accurate, easy to operate/maintain/etc & quick to acquire targets 0-300m under all lighting conditions. Not only for me...but those less capable/less familiar with weapons also. :)
Read your posting on being tired and being effective. I absolutely agree. I walked out of Afghanistan some 35 years ago through the Khyber Pass with my late wife - and I had serious intestinal problems. That is a long way from watching Rambo in a movie theater, eating popcorn and watching movie actors give each other high fives. I vowed that I would continue training my whole life - and I have. When you are into day 3 of a mantracking class or a sniper class and have been out sleeping on the ground, you gain a good understanding of your limitations and that of equipment.
It is one thing to watch a movie like Red Dawn. It is another matter to be sick, dirty, cold, wondering when you will have your next meal, watching out for someone else and looking ahead to danger.