Just as the differences between .223 and 5.56 NATO are hotly debated and shrouded in misinformation, the same applies to the world of the .300 family. The new and popular 300 Blackout round was officially developed by the Advanced Armament at the request of the U.S. Military, but the story of the 300 BLK round starts well before this.
Industry legend J.D. Jones developed what most consider to be the “first” 300 BLK round, aptly named by Jones himself as .300 Whisper. Jones developed the Whisper cartridge with generally the same intentions as what the military wanted out of 300 Blackout: A big, heavy round that could go or slow and be suppressed or unsuppressed without requiring any field modifications to the rifle firing it.
So, what’s the difference?
For starters, 300 Whisper does not follow the Sporting Arms and Ammuniation Manufacturers’ Institute standards – that’s SAAMI specifications, for most gun folks. Instead, it adheres to the Commission Internationale Permanente pour l’Epreuve des Armes a’ Feu Portatives – holy !@#$. Let’s just call that the CIP standard.
Although the two cartridges are regulated by two differing ammunition authorities, worry about – SAAMI doesn’t allow duplicate cartridges to receive certification. In this case, this is not a reflection that “official” .300 BLK is truly much different from 300 Whisper.
There’s the short of it: There is no difference
What exactly happened was this: When the military asked Advanced Armament Corp. for a .30-caliber round that could be fired from the M4 platform, using the existing hardware (bolt, receivers, and magazines), they already knew the 300 Whisper would suffice.
The problem with that was that Remington would only accept SAAMI-certified ammunition in its contract rifles, and 300 Whisper wasn’t able to receive the certification. So, AAC and J.D. Jones took the same concept, sliced apart its specifications, and submitted a cloned round to SAAMI: The .300 AAC Blackout, or just 300 BLK.
Okay, There Are Tiny Physical Differences
Yes, just like .223 and 5.56, there are some basic, physical differences between 300 BLK and 300 Whisper. For you technical reloading folks out there, you’ll find some differences in the shoulder diameter (.3618” vs. .36”), the base-to-shoulder diameter (.869” vs. .8664”), the free bore length (.18” vs. .174”) and the base diameter (.3769” vs. .3760”).
The primary measures, however, are the same: Minimum chamber length, neck diameter, freebore diameter, pilot diameter, and neck length are identical.
Thankfully, the differences are miniscule enough that Hornady has their own 300 Whisper tested by SAAMI and found it was within specifications for 300 BLK chambers and barrels.
Most major manufacturers – including Hornady – are producing reloading dies that are marked “300 Whisper/Blackout”. Smith & Wesson even stamps the barrels of their M&P-15 Whispers with “300 Whisper/300 AAC Blackout”.
What’s Safe to Shoot
To keep things simple, think of it this way: You can shoot 300 Whisper in a 300 Blackout rifle. You should probably avoid shooting 300 Blackout in a 300 Whisper rifle because of the simple fact that the Whisper round is a wildcat cartridge. Little information is available to the public about tolerances and specifications, and so you might risk running higher chamber pressures that exceed what the subject rifle could handle.
Sound familiar? You can shoot .223 in a 5.56 rifle, but not the other way around. Now you know.