7.62x39 is based on an AR-15 format, not the large frame AR like 308 or 6.5CR. It also has no more effective range in an AR than it does in an AK.
There are 5 or 6 calibers that are generally available for the LFAR...308, 6.5Cr, 243 Win, 7mm-08 and 224 Valkyrie is what I can remember off the top of my head. 308 is by far the most commonly available and usually the least expensive. There are others that have better ballistics.