....it will depend really on the chamber of the platform you will be shooting it out of... .308Win can handle both, some 7.62x51 can only handle "7.62x51", especially older MILSURP rifles due operating pressures. Most commercially available barrels are chambered in .308Win, for both bolt action and AR10's. In your stated scenario of "urban warfare", the typical engagement distance will be within 1-200M, easily accomplished by .308WIN and 7.62x51, but also many other "common" calibers. Plate carriers cover the critical central core, but there are many areas outside of the plate area that will still stop a threat, or severely hinder it's advance. With the current "shortage" of ammo availability, buying "cases" to stockpile will be very costly at current (and rising pricing). Reloading is a good contingency option. Relying on a single caliber is creating a single point of failure. If you forsee yourself in an "urban" environment should the SHTF, you might want to read up on what some of the Bosnia survivors wrote about surviving in that kind of environment.
