Interesting. As much as I love some of my less than common rounds, in hindsight, wish I would have built my arsenal around a very few common & widely available calibers & HEAVILY stockpile them. SHTF, doubtful supplies would continue to be produced so I'll base my choice on scrounging. 223/5.56 for rifle. Not my 1st choice but think it would be the more common round found in any kind of quantity now days. 2nd choice for rifle would be .308. Very popular round for hunting, should be able to find some easily, though in smaller quantities . I have several platforms & ample ammo for both rounds I mentioned but if parameter for this exercise is only choose one, I'd have to stick with 223/5.56. Lighter weapon to carry, lighter ammo to carry allowing more to be carried on person & fair chance of picking up more while pilfering.