This is quite the rabbit hole to go down, but there are lots of reasons for 9mm. Namely great ballistics, capacity, ease of shooting, and availability. Lmao @ 44 magnum. Too many Dirty Harry movies for you.I would choose a semiautomatic rifle in 308 Winchester/7.62x51mm. I do like the M1A, but there are many rifles to choose from. An AR10 would be a good choice. Barrel would be 18" long.
My service pistol would be in 10mm, so I'd choose the Glock 20.4.
Remember this is SHTF, so carrying a small 9mm semiauto makes no sense. You can conceal far more gun than you think. The 10mm gets me low to middle 357 Magnum ballistics. I could go with a revolver in 44 Magnum, which could make quite a bit of sense in certain situations. However, humans will be the most powerful predator of other humans, so the Glock 20 wins. I can always use the rifle on critters.

