Let's see: Going somewhere where spare parts might be hard to get like back in time or something:
Glock 26
Glock 18
Kel-Tec Sub-2000 carbine setup for Glock magazines in 9mm
Plus a zillion rounds of that good european subgun 9mm ammo and a variety of Glock mags, including the 31 round G18 mags. All guns use the same mag and I am covered for CCW, battle, and carbine. Sure the 9mm is not the best cartridge but with enough ammo and mags you can get a lot of holes quickly.
More realistic SHTF situation, Red Dawn scenario:
.45 ACP hicap pistol such as Glock 21, XD45, USP, etc. Trijicon sights
Mossberg 590 Mariner with Marinecoat
FN-FAL variant, probably a lightweight Para folding stock model with short barrel and an Eotech holosight.
Attack of the Zombies:
SPAS-12 or similar hicap semiauto street sweeper 12-ga
M60 auto MG with mucho ammo
Ruger Alaskan revolver, stainless, .44mag (for emergency HTH, Coup De Grace of my buddies who are bitten, or myself if necessary)
TEOTWAWKT (no more BATFE) like Mad Max:
12-ga shotty double barrel sawed off right at the end of the foregrip
Stainless steel revolver, probably the Ruger Alaskan again in .44 mag
FN-FAL variant, probably the short barrel lightweight folding Para again with Eotech Holosight, full auto capable. Or one of those new HK's with the same sight.