300 Whisper ammo was designed in the early 1990s under the guidance of J. D. Jones of SSK industries. According to SSK, "a Whisper cartridge must be capable of sub-sonic extreme accuracy with very heavy bullets for its caliber (240 grains in .30 caliber) as well as moderate to high velocity while maintaining excellent accuracy with light bullets for the caliber (125 grains at 2300 FPS in .30 caliber)."
The 300 Whisper is based on the .223 Remington and .221 Fireball cases. But, unlike its progenitors, it uses a heavier .30 caliber bullet. It is sometimes called a .300 Fireball, a .300-221, or a .300 Warrior Carbine - mainly in efforts to sidestep the fact that the "Whisper" designation is a copyrighted trademark of SSK industries. In some of its variants, it meets the same specs as the 300 AAC Blackout. In fact, it is compatible with arms chambered for the 300 AAC Blackout (depending on which designer one speaks with, they are interchangeable). Its designer specifically states that it was developed to be used in a variety of weapon designs from AR/M platforms to bolt actions to single shots, and to be easily sound-suppressed; characteristics that make it of special interest to law enforcement and military consumers. The round is very efficient and excels when used as a subsonic round in sound-suppressed arms. Of course, it is available in both sub- and supersonic offerings from several major ammunition manufacturers.