|Place of origin||United States|
|Parent case||.221 Fireball/.223 Remington|
|Case type||Rimless, Bottleneck|
|Bullet diameter||7.82 mm (0.308 in)|
|Neck diameter||8.46 mm (0.333 in)|
|Shoulder diameter||9.14 mm (0.360 in)|
|Base diameter||9.54 mm (0.376 in)|
|Rim diameter||9.60 mm (0.378 in)|
|Rim thickness||1.14 mm (0.045 in)|
|Case length||34.90 mm (1.374 in)|
|Overall length||57.00 mm (2.244 in)|
|Rifling twist||203 mm (1 in 8 in)|
|Primer type||Small rifle|
|Test barrel length: 16 in|
Source(s): CIP specifications COR-BON Ammunition
.300 Whisper is also known as .300 Fireball or .300-221. The 300 AAC Blackout is the most successful 30 caliber cartridge with these design parameters and been widely accepted whereas the .300 Whisper has fallen into disuse.
The .300 Whisper was originally based on the .221 Fireball case necked up to .30 caliber. However, reloaders have found the .223 Remington or 5.56×45mm NATO works well when shortened and resized to .30 caliber. Firing in the .300 Whisper chamber results in a slightly sharper shoulder. Magnum pistol powders such as H110 work well for supersonic loads. Sierra 240 grain (16 g) jacketed bullets work well if the barrel has a 1:8 twist. Barrels with a 1:10 twist will stabilize 220 grain (14 g) bullets at subsonic speeds. 125 grain (8 g) bullets will reach 2,400 ft/s (730 m/s).
"Whisper" is a registered trademark of SSK Industries. In order to sidestep this branding (and/or any licensing fees required to use the "Whisper" name legally), other manufacturers tend to use different names for the same cartridge, such as ".300 warrior carbine", ".300 Fireball" or ".300-221", both based on the .221 Fireball parent case. The .300 AAC Blackout has succeeded as a commercial cartridge while the Whisper never achieved wide acceptance.