That's right, same as the 7.62 NATO, 7.62x51mm. The AK/SKS round, at 7.62x39, is a lot less powerful. The M-16's 5.56 mm round does not have the M-14 round's penetrating power and long range effectiveness.
Thanks for the clarification on the cartridge length. I always forget which is which.
And while you are correct about long range effectiveness vs. the 5.56, they still penetrate pretty well, and I would submit that the GI weapons don't need to be effective any further out than the M16/AR15 is. You're talking sniper rounds/weapons, which are more likely to be chambered in the larger rounds you mentioned above (or a completely different round of a similar size). Additionally, I don't know if I like the idea of the larger round being issued to LEOs in an urban setting. Seems you'd want the lighter round for safety's sake.