In the interest of kicking off this discussion, I'd like to offer some thoughts I've had. I won't get into everything I've thought of, but I'll give some thoughts I think aren't useless.
9x19 and .22LR are extremely common, so they present themselves as strong contenders for pistol and pistol carbine/submachine gun. .30-06 is an obviously strong choice for rifle.
Because time frame is open-ended, it's possible we might desire to be able to craft our own cartridges. Brass we could keep. Gun powder... I don't know enough to talk about possibilites. Some simpler rounds that aren't dependent on complex formation (e.g. Spitzer rounds) could be easier to craft projectiles for from scrap. What about a reproduction Model 1873 (or analogue) in .44-40? Again, powder needs to be addressed, but you could reuse brass, and you could, relatively easily, mold the projectile. The mechanism of a Model 1873 is also relatively simple to maintain.
Ammunition is heavy, so sharing ammo across weapons is great. Is there a pistol round with sufficient range that it could be used for general-purpose hunting and placement defense, to such a degree that you could forego carrying a longer-range rifle? That would lower weight, reduce ammo requirements, etc.
Thinking out loud, here.