1. 5.56 AR
2. 22lr auto or revolver.
3. 12 ga, 20 inch barrel.
4. Glock in 9mm.
Anything beyond this list is hard to manage if you have to bug out.
.22lr ammo will be THE barter material for parts and other ammo, so you want an abundance. I buy anything on sale, no matter the brand because it's not for shooting.
I buy from ammoman.com. Free shipping and good prices. Bulk packaging.