300 AAC = 123 gr @ 2300 fps
Sounds like a 7.62x39 but with less case taper. Not being critical, but that is what it seems similar to.
http://300aacblackout.com/images/300BLK-556Pic.jpg
30 REM AR = 125 gr @ 2800 fps (I do not know if these published ballistics are real)
6.5 Grendel = 123 gr @ 2400 fps
7.62 x 39 = 123 gr @ 2400 fps