.300BLK forum is at http://300blktalk.com/ run by the guy who designed it.
The .300blackout was designed because the .223 just did not have the stopping power. This is a serious load. Problem right now is availability of the cartridge. Cabella’s, etc carry a variety of .223, etc but little in the .300 blackout. I get mine from a local gunshop. Bit more pricy than the .223 but it is a much stronger load, even in sub-sonic.