@olliej. I think you are reaching some interesting conclusions. However your foundational facts, are a bit off - -and that's, perhaps, why your conclusions end up off.

I invite you to review some materials with regards to

a) Difference between automatic and Semi-automatic rifle

https://www.dailyshooting.com/full-auto-vs-semi-auto-gun/

b) What considered to be more lethal effective fire patterns (it is not pray and spray afforded by fully automatic, it is burst -- but burst firing weapons are not available to US public (as they are considered automatic weapons) )

https://www.reddit.com/r/guns/comments/2yvii6/what_happened_...

c) Various approached to avoid overpenetration

(this is something widely taught to US law enforcement)

In short, AR-15 is an example of a 'modular platform'
Just like in some programming languages you can replace Garbage collector or memory allocators.
In Ar-15 you can replace barrel and bolt to leverage different caliber ammunition , and often keep same magazine.

The calibers that do not shoot as far, and over penetrate less are .300 blackout and .458 SOCOM (or wildcats like .338 Spectre). Usually they have fatter cartridge case (so one will have fewer of them in the standard capacity AR magazine). They will have heavier, often softer bullet, and will fly less distance than the standard .223 round.

Prepers, often do not just save ammunition, but also ammunition components that lets them reload a spent case with new powder + primer + bullet.

Bullet designs are a bit of a 'science' on it is own - -there are the ones that penetrate, the ones that do not , the ones that designed to mashroom, and so on.
As an example, a so called hollow point bullet will often mushroom before it over penetrates.

Ability to use multiple calibers, depending on the situation is one big reason why ARs valued more than that semi-auto AKs for example. Different calibers allow a prepper to hunt different types of game at different distances, defense, reuse neighbor's ammo, etc.

d) Prepping in concert with your neighbors (not against them).

https://www.backdoorsurvival.com/importance-of-community-and...

many in prepping communities dismiss 'single person or family compound strategy' and instead advocate an synchronous community prepping, as that's considered much more sustainable long term.

e) Theft is not a forcible felony, so lethal force cannot be used to defend against theft and 'Stand your ground laws do not apply at all.

A person who would shoot at a running thief who is stealing a car in a driveway, or a axe from your store, will end up in jail.

Forcible felonies usually include (example Florida) :

https://www.jacksonvillecriminalattorneyblog.com/what_is_a_f...

– Kidnapping

– Murder

– Manslaughter

– Sexual Battery (Rape)

– Arson

– Treason

– Robbery

– Burglary

– Carjacking

– Home Invasion

– Aggravated Battery

– Aggravated Assault

– Aggravated Stalking

-Any other felony that involves the use or threat of violence against any person

Also important that the person claiming self defense was NOT an initial aggressor.

For example, if you are at a restaurant with your spouse, and somebody is insulting her/him. You stand up and start arguing with that person, may be push him. Then he runs at you with a chair or knife, you shoot him.
You will likely be convicted and go to jail because you were aggressor as well. If on another hand you stood up, called police and started walking away from the situation, and then you were attacked and defense your self -- the legal claim for self-defence could stand (I am not a lawyer).

This is why some often say that 'Armed society is a polite society'. Legally armed civilians will rarely advance into a confrontation can be simply avoided.