It's designed to fill a niche and serve a particular purpose. The idea is a reliable AR pattern rifle that will shoot effective subsonic ammunition. Another nice plus is supersonic .300 BLK is ballistically superior to 5.56 out of extremely short barrels. The biggest advantage, however, is the ability to have a short, compact, reliable and effective semi auto AR.
The round has been around for years, it was just called the .300 whisper in the past. AAC and Remington commercialized it.
Another similar idea is the 9x39 Russian designed for the VSS Vintorez.
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