According to Long Range Shooting Handbook, SOCOM has officially chosen the .300 Norma Mag for their Advanced Sniper Rifle (ASR) Cartridge.
Here is photo comparing other popular cartridges to the .300 Norma Mag.
Here is what Long Range Hand Book had to say on the new cartridge decision.
The 300 Norma Mag is based off of the 338 Norma Mag case necked down to accept a .30 caliber bullet. It launches a 220gr .308 bullet at just over 3,000 fps. That’s 4,400 ft/lbs of energy! This cartridge is not to be confused with the 308 Norma Mag. The 300 Norma Mag really shines when it is shooting either the 230gr Berger Hybrid bullet – this bullet has an out-of-this world 0.743 Ballistic Coefficient (BC)! (If you’re looking to learn about BC, check out Chapter 10 of the Long Range Shooting Handbook) When loaded up to 3,000 fps, this bullet from the 300 Norma Mag will stay supersonic out to 1,500 meters! This is as far as a much heavier 300gr SMK shot out of a 338 Lapua can make it before it goes subsonic! Even though the 300 Norma will only have 80% of the energy of the 338 Lapua at that distance with those bullets, it can still make it there accurately with considerably less recoil. Another benefit to the 300 Norma Mag is the overall length is shorter than the 338 Lapua Mag – it is effectively the same length as the 300 Win Mag. Cleckner.co
Despite its benefits, it is still an interesting choice. For example, it is based off of the same parent cartridge as the 338 Lapua mag….the 416 Rigby. This means that a bolt with a large face is needed which often means a large action. It is nice that it isn’t a rimmed magnum like the 300 Win Mag (I’m biased against rimmed cases), but I’m not yet convinced that the performance necessitates a larger bolt face and action. For example, when loaded with the same bullet, the 300 Win Mag, 300 RUM, or 30 Nosler don’t fall that far behind the 300 Norma Mag. In fact, there’s less of an energy difference at 1,000 meters between the 300 Norma and the 300 Win Mag (the weakest of the 3 alternatives) than there is between the 300 Norma Mag and the 338 Lapua Mag. And, each of the alternatives listed use a smaller bolt face and a standard “magnum” action.
I’ll be interested to see why this particular cartridge was selected. If I could, I would’ve bet on the 30 Nosler but I would’ve lost.