[Mike Bell]

Smith & Wesson's 300 is only different in that it's chambered in 300 Whisper, a wildcat cartridge that the 300 BLK is remarkably similar to; it is only shorter in overall length, which means that 300 BLK can be safely and accurately fired in a gun chambered in 300 Whisper, which is ballistically identical but a little more expensive

http://www.guns.com/smith-a-wessons-...-blackout.html