I think in a post SHTF scenario, we're going to best be served by having firearms and ammo setups that are simple, rugged, and reliable. While zombies are still infiltrating your perimeter every day, it would behoove you to have as few complications between you and the bang switch as possible.
This is why I own multiple weapons of the same caliber, and it's why I keep those calibers restricted to the most common varieties around.
'Keep it simple' is very often the best approach. However, life is not always predictable (if it were, we would all have invested every penny we had in a winning stock, and be retired right now). If you go "all in", and there is no alternative (no 'Plan B'), it is possible to find yourself standing there, asking, "What do I do now?"
(Ask me how I know... ;^)