The .300 Whisper is a CIP standard cartridge in the Whisper family, a group of cartridges developed in the early 1990s by J.D. Jones of SSK Industries. It was developed as a multi-purpose cartridge, capable of utilizing relatively lightweight bullets at supersonic velocities as well as heavier bullets (200–250 grains) at subsonic velocities.

.300 Whisper is also sometimes known as .300 Fireball or .300-221, but the .300 Whisper is a CIP cartridge and other names are generally known as wildcat cartridges. When compared to .300 AAC Blackout Steve Johnson’s article in American Hunter sums it up best:

Given the fact that major manufacturers such as Hornady are building ammunition headstamped .300 Whisper and recommending it for use in either platform—in addition to producing reloading dies that are marked “300 Whisper/Blackout” and Smith & Wesson stamps the barrel of its M&P-15 Whisper barrels with: “300 Whisper/300 AAC Blackout” it is safe to say that differences are minimal.

The CIP has standards for both .300 AAC Blackout and .300 Whisper defined. The two are very similar in these definitions.