Try these to start:
http://www.300aacblackout.com/
http://en.wikipedia.org/wiki/300_AAC_Blackout_%287.62x35mm%29
I have no hands on experience with it. From what I can tell the .300 Blackout replicates the 7.62x39 performance with 123 gr bullets, maybe a little faster. It has some advantages over that however:
- Little body taper so it can use standard USGI 5.56 mags and does not have the mag/feed issues of the 7.62x39 in the AR15 platform
- Uses the more numerous and varied .308" bullets rather than the .311"
- Can shoot the 200/220 gr bullets subsonic for more effective suppressed fire, as well as 123 gr bullets at around 2,400 fps from a 16" barrel for hunting or self defense. From short barreled rifles it has better performance than the slow 5.56 which loses a lot of velocity in SBRs.
So it has the better terminal ballistics of the 7.62x39 bullet weight, can be subsonic or supersonic, feeds from standard USGI mags with no modifications, and uses common .308" bullets.
On the down side, it has the same range limitations of the 7.62x39 of not much more than 150-200 yds before the bullet drop becomes significant.