The .300 Whisper, displayed center.
|Place of origin||United States|
|Parent case||.221 Fireball/.223 Rem|
|Case type||Rimless, Bottleneck|
|Bullet diameter||0.308 in (7.8 mm)|
|Neck diameter||0.330 in (8.4 mm)|
|Shoulder diameter||0.369 in (9.4 mm)|
|Base diameter||0.375 in (9.5 mm)|
|Rim diameter||0.375 in (9.5 mm)|
|Rim thickness||0.041 in (1.0 mm)|
|Case length||1.35 in (34 mm)|
|Overall length||2.10 in (53 mm)|
|Primer type||Small rifle|
|125 gr (8.1 g) JHP||2,100 ft/s (640 m/s)||1,224 ft·lbf (1,660 J)|
|220 gr (14 g) JHP||1,040 ft/s (320 m/s)||529 ft·lbf (717 J)|
Test barrel length: 16 in
Source: COR-BON Ammunition
The .300 Whisper (also known as .300 Warrior Carbine,.300 Fireball or .300-221) is a wildcat 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.
The .300 Whisper was originally based on the .221 Fireball case necked up to .30 caliber. However, avid reloaders have found the .223 Remington or 5.56×45 mm 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 subsonic 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 2400 ft/s (730 m/s).
There are a few points to be considered regarding the utility of the .300 Whisper:
In addition to special applications in suppressed firearms, the .300 Whisper has become popular with metallic silhouette shooters due to its low recoil, good long range performance, and very high accuracy.
"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.