There has been a round known as the 7.62x45 which is a light machine gun round for the Czechoslovakia Gov't. It pushes a 130gr bullet at 2500'ps. The 308 Arrow is a blown out and slightly stretched 7.62x39 round. It pushes the 125gr bullets at 2750'ps and 110gr at 2900'ps. Since it's being built on an AR platform, it probably will die a slow death IF it ever gets off of the ground. Clip capacity will be limited just like the 30AR Remington only the Remington gets you about another 200'PS. Don't need the extra 200'ps, then there's the 300 Blackout that pushes the 110s at 2400'ps and the 125s at 2200'ps with the ability to go completely subsonic with 220gr bullets. While I would love a cartridge based on a small case that I wouldn't lose clip capacity in an AR platform, frankly you run out of HP in the small case before you can get into any large caliber. The biggest that is still effective based on the 223 case that I've seen is the 25/223. I just finished a 6mm/223 and so far LOVE it. Accuracy has been numerous one hole groups at 100yds with bullet weights from 70gr to 90gr. The 90's peak out at 2900'ps out of my barrel with a compressed load of Varget.
