The .300 Whisper was developed by cartridge wildcatter J.D. Jones, owner of SSK Industries. Formed by simply necking up the .221 Fireball case to .30 caliber, the .300 Whisper was designed so that it would launch heavy bullets slower than the speed of sound; there would be no "crack" caused by a sonic boom.
The .300 Whisper was designed to be compatible with the AR-15 platform. You simply swapped uppers, loaded mil-spec magazines with the same number of .300 Whisper rounds as 5.56 NATO, and fired at will. Adding a suppressor gave you a potent law enforcement or military asset.
At the time, ARs were not on my radar, but working with a new and interesting cartridge was right up my alley. It wasn't a SAAMI standard round, and back then only CorBon offered factory-loaded ammo.
I set up to handload the .300 Whisper with a set of Hornady dies and some new .221 Fireball brass. The special tapered expander die that came with those dies swiftly and smoothly reformed the cases. As I recall, I then just full-length sized them and added primer, powder, and bullets. I wasn't into suppressed ARs, so I limited my experiments to lighter bullets traveling at supersonic velocities.
Commercial and military-surplus .223/5.56 brass was much more readily available, so I also reformed those cases to load for my Contender. Of course, that process is more difficult and time-consuming, but the resulting .300 Whisper cases were almost always usable.
I did need to carefully adjust the sizer die in order to achieve correct headspace. I'd read about having to turn necks when reworking .223/5.56 cases, but my handloads would always chamber and fire reliably without having to do that. I also didn't anneal the case necks.
After experimenting with the Whisper for a short while, I moved on to other new and/or interesting rounds. But I kept up with industry events and heard about a new cartridge called the .300 AAC Blackout when it first surfaced. Developed specifically for military purposes, it is a SAAMI standard cartridge. However, for all practical purposes, it is identical to the earlier .300 Whisper design.
Among the many products shown these past few years was a Smith & Wesson M&P15 carbine chambered in .300 Whisper/.300 AAC Blackout. I ordered it and a Nikon M-300 optics package specifically configured to complement this cartridge's unique ballistics. I used that combination along with my T/C Contender to work up some new .300 Whisper handloads.
As you can see in the chart, I tested quite a few handloads with bullets weighing from 110 to 200 grains. I even tried a couple of near-subsonic recipes. The M&P15's 1:7-inch twist rifling handled them just fine; however, the slower 1:10 twist Contender barrel wouldn't stabilize the heavier bullets, especially at subsonic velocities.
I fired the iron-sighted Contender at 25 yards and the M&P15 at 50 yards to start, but after I'd fired 100 or so rounds in the rifle, I moved out to 100 yards and fired a few select loads. Not knowing just what to expect, I thought 2 MOA was acceptable performance for this cartridge/rifle/optic combination.
Easier to Handload Today
Today, handloading the .300 Whisper/AAC Blackout is no different than most other bottleneck rifle rounds. Brass is readily available, so tediously reforming .223/5.56 cases is no longer necessary. I should mention also that Barnes, Hornady, Remington, and possibly other ammo manufacturers offer factory loads in .300 Whisper/.300 AAC Blackout, and I was able to purchase several boxes of it at my local gunshop.
I tumble the cases and brush the case necks before full-length resizing. Unless you want to crimp the bullet in place, trimming your brass is optional. Hornady's 140-grain MonoFlex bullet has relief grooves formed in the shank. I trimmed the cases for these handloads because I intended to crimp the case mouth into the rearmost groove. It probably isn't needed, but the seater die can be adjusted to apply a crimp - so I did. These loads performed quite well.
So, what impressions have I gained with my renewed experiments with the .300 Whisper/.300 AAC Blackout cartridge? I still have no experience with subsonic ammo fired in a suppressed rifle, but common sense tells me whatever is on the receiving end of a 200-grain bullet traveling at about 1,000 fps is in big trouble.
I have fired enough supersonic loads to determine that the .300 Whisper would be an excellent, albeit short range, varmint and deer cartridge. The 110- and 125-grain handloads would readily dispatch any coyote or chuck within 150 yards or so, and the 130-, 150-, and 168-grain ammo, properly placed, is just powerful enough to take a porker or whitetail.
I do see two reasons to emphasize the round's limited range. One, the Whisper bullet's muzzle velocity is similar to the remaining velocities at 100 to 150 yards of other, more common hunting cartridges. Obviously, the Whisper's residual energy available at 150 yards is correspondingly reduced. Hunting bullets require adequate energy to expand and kill reliably. Two, the Whisper's lower muzzle velocities result in greater vertical bullet dispersion, i.e., the slower bullets drop more at extended ranges. That means you have to be even more attentive to knowing your bullet's actual trajectory to be sure you will strike your intended point of aim.