I think most of us would agree that we'd like something between the .223 and .308 for a service rifle. I know I would. Our Special Forces folks have been working on a 6.8 mm round, I believe, and it sounds like a good idea.
The military has been ordering a lot of 6.8 x 43 ammunition. The round really packs a punch out to about 500 meters from a sixteen inch barrel. I have heard that they are going to make the switch from 5.56 x 45 to 6.8 x 43 within the next ten years. Phase it in eventually.
Until then I really hope they start issuing the 77 grain 5.56 x 45 to our troops. It has about 100 meters more effective range than the 62 grain 5.56 x 45 ammunition.