For a first centerfire rifle, especially if it is expected to be the only centerfire rifle in the collection, 308 is really hard to beat. It is such a versatile cartridge, with a large ammo variety.
Why the interest in armor piercing ammo? Are you thinking you need it to hunt dangerous game? Not so. Both FMJ and round-nose solids will penetrate 36 inches plus of muscle and bone. FMJ spitzers usually turn base forward after a dozen or so inches of penetration, and from that point they act as a wad-cutter. Round nose solids are the best, they penetrate straighter and deeper than any other.