One in particular was the twist rate of the barrel. In the early 90s a 1:10-inch twist barrel was about the fastest that could be found in .30 caliber. Jones discovered that the .300 Whisper does best with a 1:7-inch twist barrel. This is all the stuff that comes together to make the .300 Blackout awesome. May 02, 2019 · If you aren’t familiar,.300 Blackout, or.300 BLK, is a round that is designed to work with the AR-15 platform, and to excel in short-barreled and/or suppressed weapons. It mimics the Russian 7.62×39 round, and can even be loaded into standard 5.56/.223 AR-15 magazines.