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This is a question I’ve been asked a lot recently. Chaz, a commenter on TTAG, asked it this way:

The 300 Whisper and 300 Blackout have small differences in their brass dimensions: Blackout 0.004 larger in the neck, 0.018 longer case length and 0.16 longer OAL. Also the Whisper is a wildcat while the Blackout is SAAMI standardized. Apparently those differences make the Blackout the more attractive of the two.

Would a Blackout chamber accept a Whisper round? How dangerous might that be?

I posed that same question to the guys at Advanced Armament Corp., the skunkworks that developed and is producing the .300 AAC Blackout round, and got a fairly quick response…

I quote John Hollister, the bearded wizard of Advanced Armament:


300 AAC Blackout and 300 Whisper ™ are not the exact same thing. It is similar to .223 vs 5.56mm in that the difference is in the neck of the cartridge.

300 AAC Blackout is SAAMI spec cartridge whose origins come off the 5.56mm case and cartridge case drawings and chamber drawings are available from SAAMI for the round and in fact are available to anyone right on line. [Click here.]

300 Whisper ™ is a Wildcat cartridge based on the 221 Fireball case and there is limited information out to the public or manufacturers in the way of cartridge drawings or chamber drawings. 300 Whisper ™ has traditionally been a round oriented toward handloaders and there is a lot of reverse engineering done out in the marketplace. There is not a published “Standard” for 300 Whisper ™ chambers and cartridges. As such, we cannot tell customers that you can shoot 300 AAC Blackout ammunition in a 300 Whisper ™ chambered firearm, or that you can shoot 300 Whisper ™ ammunition is a 300 AAC Blackout chambered firearm.

Our recommendation is that if a consumer want to cross ammunition and chambers with these two caliber, that they have the ammunition and firearm checked by a qualified Gunsmith to assure it can be done safely.

John Hollister | Sales Manager
Advanced Armament Corp.

If I’m reading this right, basically what John is saying is that because the .300 Whisper round isn’t a “standard” cartridge the chamber pressures of a particular ammunition might be too much for a .300 BLK chamber to handle. Might. Possibly. Depending on the ammunition and its manufacturer. The lack of a proper specification for .300 Whisper means that some ammunition may be loaded “hotter” than normal and still be called .300 Whisper, leading to an unfortunate surprise that could blow up your gun.

Thanks to that caveat it does appear that firing .300 Whisper out of a .300 BLK firearm is possible if you get the right ammunition, as John’s closing paragraph seems to indicate. .300 Whisper WILL fit in a .300 BLK chamber (assuming the cartridge is loaded to spec), the only question is whether it will blow up on you when you pull the trigger.

The best method to avoid an explosive and bloody death when using .300 Whisper in a .300 BLK gun is to load your own ammo. And honestly, if you’re already loading .300 Whisper it’s $30 worth of reloading dies before you can swap to .300 BLK. By loading your own rounds you can ensure that your maximum load never exceeds the recommended maximum load for .300 BLK and thus be able to use the same ammunition in both firearms.

The second best method is to find a manufacturer that loads .300 Whisper ammunition to also meet the .300 AAC Blackout specification. Hornaday, for example, loads their .300 Whisper ammunition with a careful eye to not exceed the maximum chamber pressure for .300 BLK and so their ammunition (according to the very nice tech guy on their customer service line) will function in both .300 Whisper and .300 AAC Blackout firearms. I’m sure there are other manufacturers making .300 Whisper that do this, you just need to make sure that they explicitly state that their ammunition will not blow up your gun before you go trying it.

In summary, yes you can fire .300 Whisper out of a .300 BLK firearm IF the ammunition was designed to do so. Hornaday’s ammo is the only one I investigated, but I’m sure other manufacturers are following suit to increase the marketability of their product. I still would take the gun and ammo to a gunsmith for testing or use the “long piece of string and vice grips” method to test whether it really is safe before trusting it to go off so close to my face.

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  1. Unless you reload and have a understanding of whats going on, you’ll probably want to avoid doing so. I have a 40 Super conversion for my FNP-45 from EFK Firedragon, the chamber’s neck is aprox 0.015″ shorter than the spec. I have 2 cartridge drawings from Triton’s ex-CEO/founder which differ from each other, and QuickLOAD has it own cartridge drawing which differs from Triton’s as well. So there are at least 4 different dimensional specifications out there for 1 obscure cartridge.

    When you look in to pressure, DoubleTap rates their brass for 40k PSI. I contacted Starline which also makes 40 Super brass, and they said “Our .40 Super brass has not been rated. It will stand up to, and exceed, pressure levels of factory ammunition.” The problem with that is that they made the brass for Triton, who never standardized the pressure.

    If anyone out there knows about 40 Super, let me know, i’d like to talk to you about your reloading experiences, or purchasing Triton ammo if you still have some.

  2. May have been answered but can I safely shoot 300 blackout in my m&p upper. Haven’t had any probs so far.

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  4. I’ve got a potential ID 10 T question here. If we are taking about subsonic ammo – does a “hot load” even present an issue? I could see a supersonic load, but we are talking about a round that has already been dialed down to a lower muzzle velocity. I haven’t reloaded since I was a young teen with my dad and unfortunately much of that knowledge was ummm… liquefied, but I believe when we built hotter loads in for our pistol and maybe some rifle rounds, it was to eek out a little more range. I think I recall us building hotter hand loads for a .357 my dad used to hunt in the Big Thicket in East Texas.

  5. I was wondering the same thing. Just have to find out what hornady loads in their 300 whisper rounds.Very infromative article.

  6. Easy solution shoot the proper ammunition in the proper chamber.Do not take chances with your looks or don’t want to see what you would look like if your firearm blew up in your face!!!!

  7. If you watched that video of the .50 slop round malfunction in that single shot .50 kit gun that ex cop in Kentucky had a screw on sear rifle, and the shell before he almost bought the farm round threw out a huge muzzle flash. And then he nearly got killed when that last $100 a round ammo blew up the rifle. He was shooting at a fire hydrant trying those and some other AP/ incendiary shells. The sear stripped off ALL of over an inch of threads on that part of the rifle. I’d look up and watch that video before getting too brave interchanging similar sized rounds. If you just search .50 cal rifle blows up video it should come up. The footage is edited for the carnage content… and still gave me chills watching this one. Be careful. The guy really got hurt bad.

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