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|
|Test barrel length: 16 in
Source(s): COR-BON Ammunition
The .300 Whisper 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.
.300 Whisper is also sometimes known as .300 Fireball or .300-221, but there is some variation in dimensions and pressure between these cartridges.  Some implementations of .300 Whisper meet the SAAMI specifications for 300 AAC Blackout.
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 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 2400 ft/s (730 m/s).
There are a few points to be considered regarding the utility of the .300 Whisper:
- Supersonic loads are capable of matching ballistics of the 7.62x39mm and the .30-30 Winchester, making the .300 Whisper very capable as a short range deer hunting cartridge.
- The use of heavy bullets, along with the low powder weight and small case capacity, make the .300 Whisper ideal for use with sound suppressors. These subsonic loads offer energy levels similar to that of the popular .45 ACP pistol cartridge, but range is substantially increased due to the longer, more efficient .30 caliber bullet.
- Cases for the .300 Whisper can be formed from common and plentiful .223 Remington brass by sizing and trimming to length. However, J.D. Jones has recommended against this, citing the difference in case thickness at the neck (compared to the .221 Fireball) as a potential issue. This problem can be exacerbated when military 5.56x45mm cases are used. (The Whisper name is trademarked thus: the only firearms with a .300 Whisper chamber are those from J.D. Jones and Thompson Center, which licensed the name. All other guns are cut to .300/221 which has different chamber specifications than the Whisper, particularly in neck wall thickness. Because the .300 Whisper is a wildcat cartridge there were no established Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) specifications for the chamber reamers, and the necks were cut to allow for standard .308 neck thickness, which is approximately the same thickness as produced from forming .223 brass. Typically only guns with Whisper chambers will have problems with .223 brass—.300/221 chambers work very well with .223 brass, and the .223 brass is considerably stronger than .221 brass.)
- Perhaps most importantly, any firearm with a .223 Remington bolt face can be converted to .300 Whisper by re-barreling the action. As such, the .300 Whisper works well in AR-15–based rifles, especially when some type of gas regulator is installed.
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.
- List of rifle cartridges
- 300 AAC Blackout (7.62x35mm)
- .30 HRT
- .338 Spectre
- .458 SOCOM
- .450 Bushmaster
- .50 Beowulf
- .50 Hush Puppy
- SSK Industries
- Quarterbore's .300 Whisper Pages
- .300 Whisper description from AR15barrels.com
- Case dimensions from Ammoguide.com