Umm, it's a .30 bullet loaded in a necked out 5.56 case either way, and available subsonic or supersonic, either way.
Originally, it was the AR community's answer to being shut out of three gun competition. It was around before the first SOF match at Columbia, MO, and excluded there, too. Not a proper battle rifle, you know.
From there, it found some usefulness with the well heeled suppressor crowd. It does a good job subsonic, in comparison to say, a 10" in 5.56. As in, 240gr vs. 55. Of course it carries more energy and range, it can absorb more because of the exposed area of the base, and more mass. In a velocity restricted application, bullet size is the answer.
It does ok supersonic, but in that situation, it's powder capacity limits it. Other cartridges have more range and power from the same length barrels. For that matter, a .30-30 Winchester does. The .300 Whatever is ok, but not the optimum choice then.
What getting it approved as the ".300 Blackout", or .300BO as I like to call it, has done is allow other makers to sidestep the licensing fees of the Whisper originator, same as Les Baer did when "creating" the .264LBC - otherwise known as 6.5Grendel. And Alexander Arms did the smart thing by getting 6.5G made SAMMI, too. Now, the industry will mostly call it that, barrels will be so marked, and so will the ammo boxes.
It's all about marketing. A .30 bullet in a 5.56 case by any other name can't create it's own laws of physics. Case capacity gets more restricted with heavier bullets, a 240gr bullet barely has 9 grains of powder, the 125 bullets get 15.
6.8SPC and 6.5G will get 27-29 grains with 90 to 130gr bullets, and easily exceed 2,400fps. Hot 85gr loads in 6.8 have been consistently recorded over 3,000fps out of 16" barrels, and milspec 5.56 does that from 20".
The .300 won't ever see that from 130 grain loads with 10gr less powder, it's a compromise load. Subsonic, it's deliberately downloaded under 1,100 fps, which is why the bullet weight is used to make up the difference. If anything, if I was cage fighting feral hogs, the .300BO would be a welcome help. As an combat or hunting load, the military and public prefer something different.
