+1 on that .357 lever / revolver combo suggestion. You get a reliable and powerful sidearm that you can conceal or use as a backup to the long arm, and a reliable long arm that has lots of power and decent range, and has the best repeating speed after a semi-auto...all in the same cartridge. That is key. You can minimize variables, and thus spend more money and effort on stockpiling MORE when you simplify.
If you hate revolvers and levers...you can substitute .357 with 9mm, and do a semi-auto 9mm carbine, with a semi-auto 9mm pistol combo. Not as powerful or reliable, but just as good practically speaking. One big advantage is that 9mm is going to be, without a doubt, the absolute most likely easy to find cartridge of them all. Much more so than .357. That's in theory....
However....
I don't know if I am so convinced anymore about maintaining ammunition compatibility with society. Ammunition compatibility between personally owned firearms is a good idea on the basis that you have only 1 thing to carry and weight is a huge concern...but thinking about what will/won't be available outside of my immediate possession is something I no longer plan for. It is pointless, here is why.
In any SHTF, even minor ones...the ammo FLIES off the shelves. If it doesn't it will be quickly stolen/looted in the aftermath. We have testimony of this from Katrina victims right here on THR. Forget about ever seeing a single ROUND of ammo in any store within 600 miles. That means the ONLY source for ammo, is either stealing it, looting it, or picking it off a dead body (or maybe bartering for it). Stealing/looting it is not a good idea as that makes you a badguy, and it is dangerous. Finding it on a dead body isn't likely to happen, as in most realistic SHTF scenarios..you won't be encountering enough dead bodies to where there will be enough of them so that the odds have it that the one you stumble upon has ammo and in your caliber. Furthermore, whoever made that body dead in the first place probably looted the wallet/money/weapons. The only other way, is if YOU are the one who makes that body dead. That means you would have been in a firefight with an armed adversary. Not likely to happen. And if it does..what are the chances it is the same cartridge as yours? Would you even want to use their ammo? Who knows where it came from or what condition it is in? Is it some garbage reloads with a misfire or a kaboom in the waiting? Will it jam your firearm? I know what many people are thinking...they want ammo compatible with what the JBT's will be using so they can pry it off their corpses after a successful ambush -- to that I say you've got an interesting SHTF strategy
So in short - scavanging of ammo is not likely, not safe, and not a good idea even if you can do it.
Thus, pick any cartridge, even if slightly less popular...and stock pile it to satisfy your needs.
That being said, the .270 isn't a bad idea from an ammo availability perspective. Although, I'd prefer something with better rapid fire capability than a bolt-action. Rule #1 - be PREpared. That means before the disaster. If you wait, even till the week of an incoming disaster (like a hurricane)..you're out of time as people are buying things up, and if it isn't bought up, the owners employees will take it, and if they don't take it - they'll hide it or move it so it doesn't get stolen or looted. If the disaster has no warning...you're S.O.L. The #1 worst strategy is to assume that you'll be able to find resources (medicine, ammo, food, water) during or after the disaster.
Me personally? What would I choose as the ultimate SHTF rifle...given that I'll stock up my own ammo - the best choice would clearly be an AK-74. The pure robust/reliable nature of the AK, but with the much lighter ammo, mags. Practically speaking, more effective than pistol caliber carbines, as practically effective as a 7.62x39...but lighter. Can take game with it, can take out gangs of thugs. Perfect. Too bad 5.45 is simply not available to even stockpile. The other option would be a .223 AK, but one with 100% native 30rd .223 magazines made specifically for that rifle. Not converts or workarounds. Either option should have tritium irons as a must. Above that, you could add reddot, that is optional.
