M-
OPTION B: Spent .223 cases can be collected and cut down to 300. But then there’s the primer and bullet inventory thing.
See option A: common calibers means not only available ammo, but available guns and spare parts.
.223 and 7.62x51/.308 remain compelling choices.
Total agreement.
In the south, a 30-30 lever gun and a .243 bolt gun also approach universal availability.
And .243 / .308 essentially use the same brass, necked down.
Never ever buy an AR-15 in .223. It will jamb with 5.56 ammo. Buy a 5.56, it will handle .223 without a hitch. The guns/ammo are not fully interchangeable like the .308/7.62x51 are.