The 69 grain SMK was the first real target bullet for 223 and 5.56, IIRC, but its ballistic coefficient is lower than that of a flat base 150 grain .308 diameter M2 Ball FMJ bullet. Same with the copies by other companies at this weight and the Hornady at 1 grain less weight. You have to go to the 77 grain SMK to match the M2 ball bullet. The 80 grain SMK matches the BC of the 168 grain .308 SMK, and will, therefore, perform as well against wind, but it needs to be loaded long and singly in the AR to achieve a performance match. If loaded short to fit a magazine, it eats up powder space and loses some velocity. The even longer 90 grain SMK has a very good BC of just over 0.5, but it's so long you can't fit enough powder into a 223/5.56 case under it to get enough velocity to let it shine.
It's just easier to get to a high BC with the greater mass of the 308, and the BC is what tells you how the wind will treat you in any given velocity range.