First off, Welcome to the FTF Community. Some of the regulars might like you to stop by the Introductions and tell us a little about yourself. I will leave that up to you.
So, an aspiring writer and you want some weapons advice?
This world will be both Western and Sci-Fi? Will you have a group or a couple of lone gunslingers??
Your people will need a mix of short and long guns.
One piece that I think would be cool would be a Schofield top break revolver. Customized with ivory grips, stainless, and a fancy leather crossdraw rig.
Who will the enemies be in this alter universe? Other survivors who are looking to take what little your team has? Mutants? Aliens?
I'm asking because the "type" of adversary would dictate what I personally would look for in weapons.
Let's say my enemy is like zombies, driven by lust for fresh meat, but attracted to sound. Obviously I would want a crossbow, a couple of silencer equipped rifles or sub machine guns and large caliber pistols as a last resort.
Now, if noise isn't an issue because it's going to be fight or flee at every encounter?
I would pick something epic like an H&K 417 with a 100rd drum mag, twin 1911's customized for me and a room clearing shotgun.
Give us some vision of what you want and we will see if we can help.