Merely attempted to point out that there are useful ways that common ammo can be used in multiple cartridges bearing firearms. There are also auxillary type devices to fire sub calibers also. After all, a handgun is primarily used to get to a rifle! With a battle rifle, it's nice to have a bolt or another action should your primary go down, and you have a bunch of ammo, and nothing to use it in. I also would not rely on a single handgun. I favor the Glocks due to parts available, and simple to replace parts for the most part without a trip to skilled gunsmith. I own a few Glock 22s, and a sub compact that pretty much due most everything, without malfuctions at the wrong time.
An example of multiple firearms for single caliber, is what I consider a short range, as the .223 doesn't always deliver. A couple of ARs (one HB with scope and one standard Bushmaster Patrolman), A Remington bolt run w/Pac Nor barrel, a Contender (Scoped) in .223.
A step up, a Garand in 7.62 X 51, Remington 788 .308, Savage 99 lever 308...
Most of my firearms are scoped due to aged and poorer eye sight. Almost a lifetime of scoped rifle use makes for a more skilled faster use over the open sights. I am considering a few with laser and dot sights for closer than I would like a threat to be. Not real cool with most being battery dependent, so call them an option rather than a primary.
I have thought of other weapon types like bows and crossbows, but limited ranges, and a high skill level, makes it not much of a consideration for me. Black powder, again range, maintenance, noise, and powder signature, only if it was all I had a consideration. Damper climate where I live would also be iffy on operation.
Thought some on many options, but return to those that truely work each and everytime. The ARs can be iffy in the field with dampness. Won't argue that fact, as years back they stopped functioning in Nam, and a few years ago, during an Appleseed shoot in two days of rain, all four ARs had feeding and extraction problems along with a blow back .22 LR. All the other milsurp and sporting rifles had no incident of functioning problems. I guess now they use mostly .22 LR and save the good centerfire ammo for a rainy day special occasions!