Here is a photo showing how .300blk can be chambered in a .223 chamber. We have seen many examples of the aftermath of this scenario once the shooter pulls the trigger. This cut away shows how there is enough room for the cartridge to be chambered. Just like safe firearms discipline one should pay attention to what ammo is going into the gun. Perhaps this image may help explain it to you or someone you know.


  • Martin Grønsdal

    I am an operator and I never make a mistake. This isn’t a problem for me, nor my rifles.

    • Brocus

      you look and sound a lot like Karl Pilkington

      • Martin Grønsdal

        I am Carl Operator. I operate every caliber to perfection.

        • Twilight sparkle

          I think you mean operational perfection
          Perhaps the mistake is due to lack of operator beard?

          • Martin Grønsdal

            The beard melted during one of my operations.

          • Charles Applegate

            It melted off from the heat of re-entry during an extra high-speed low-drag operation, didn’t it?

          • Spencerhut

            Yeah, had that happen on one of the many times I was door gunning on the Space Shuttle and we got hit. Re-entry heat fries the hair.

          • Martin Grønsdal

            Yeah. I dislike that as well

          • Feneris

            This thread makes me chuckle, operationally.

    • pithy

      …………okay, then. I was unaware the “operator” title completely voided any human fallibility.

      • JSmath

        Well, now you know better.

      • Martin Grønsdal

        Only operators understand that.

    • AC97

      You’re a tier one operator operating operationally.

      • Martin Grønsdal

        I define tier one

    • Martin Grønsdal

      the point with my post was, besides the sarcasm, to make one point: if the hunt for new calibres always will demand compatibility with the AR15 platform, then in the end we will end up with calibers that are too similar to call it a development.

      then again, I understand that no one want to invent something that is technically better, but that no one will use/buy.

    • Anonymoose

      Well, you could mark your uppers, mark your mags, and actually pay attention to what you’re grabbing and putting in your gun. I’m sure someone somewhere has mistaken .380 for 9×19 or .223 for .222, and vice versa.

      • Martin Grønsdal

        But first to develop a brain?

  • Joe

    Thanks for illustrating the issue, I was having a hard time understanding how a .300 could chamber in a 5.56.

  • BattleshipGrey

    This photo has needed to happen for a while. We’ve gained a lot of new shooters and gun owners in the last 8 years, but that doesn’t mean they read up before dropping the hammer on the latest and greatest.

    • Jim Slade

      …and some of them are studying up by watching tactitards shoot at lawnmowers packed with tannerite on YouTube!

      • Charles Applegate

        Suddenly, I find myself wondering if a lawnmower blade would set off tannerite…

        • Cal S.

          NO, DON’T DO IT!

          Well, ok… But at least film it!

        • Risky

          As long as it’s going fast enough! I’m sure there’s a formula for the speed of rotation at a specific distance from the center of a circle based on the RPM out there.

    • ExMachina1

      Agree that it’s very illustrative. But it’s also sort of misleading in that the 300blk won’t simply DROP into a 5.56 chamber. It needs to be forced.

      • the force of the buffer spring in an AR15 is often enough to “force” it into battery.

        • ExMachina1

          That’s right but the photo makes no mention of this being specific to an AR15. That’s why I said it was misleading. Makes it look like 300BLK simply fits…

  • Jim Slade

    Oof. When I was a kid I worked in a country tire shop where they would show you why you shouldn’t mount a 16.5″ tire on a 16″ wheel by introducing you to the old boy with only three fingers on the hand he leaned on the tire machine with.

    • iksnilol

      Ummm… as someone who doesn’t mount tires. What happened?

      • Jim Slade

        They’re close enough in bead diameter that you can get a 16.5″ tire onto a 16″ wheel just fine, but when you air it up, the angle on the inside of the bead where it would seat and lock to the wheel is different and it can climb right up and off of the wheel…. likely after you’ve put 50-60 psi of air into it. Think of the air volume in a big-ass truck tire at 60 psi, and you’ve got a nifty big-ole’ kinetic energy bomb. Worst case, everything busts and sends pieces of wheel and tire flying, best case the popped tire and wheel stay together as one unit but climb off the clamps holding it to the machine.

  • USMC2090

    I once fired a 300blk from my hk416 post sample machine gun. Other than blowing out the magazine from below and breaking the bolt extractor the gun was fine. In fact the round went down the barrel without ruining the barrel. I use the weapon to this day for events…I guess that’s a testiment to the 416’s robustness!

  • Frank Grimes

    I avoid this by not using gimmicky crap calibers to begin with.

    If you were old enough to own a rifle before .300Blk existed but didn’t own one in .300 Whisper, then .300Blk comes out and you think it’s the greatest thing ever, you fall for gimmicks.

    • Duray

      Being SAAMI approved doesn’t make something a gimmick. Not everyone wants a proprietary round.

    • Peter Marcus

      300 Whisper was not a SAAMI round, and not very reliable. 300 BLK fixed all of those issues.

      • Frank Grimes

        It’s virtually no different. Any lack of reliability is a user issue or just poor quality hardware issue.

        5.56mm NATO is not a SAAMI round either, I hope you don’t shoot that.

        Wouldn’t want to run into something being “not very reliable”.

        • Peter Marcus

          5.56mm is absolutely a standardized round as per NATO specs.

          AAC and Remington with 300 BLK solved the Whisper relibability problems by Remington creating custom long-ogive bullets, loading them to a specific OAL such that the part of the bullet that is the same diameter as a 5.56mm case neck rests on the magazine rib, and selected powders and gas port sizes that allowed for subsonic and supersonic operation without the need for an adjustable gas block in AAC rifles. But yes, plenty of companies have made ammunition with poor bullet choices as in this article.

    • RocketScientist

      you’re a f***ing idiot, Frank.

      • Buddy_Bizarre

        Can’t blame him, he has to work with Homer Simpson…

      • Frank Grimes

        Lookie here guys, we got somebody who fell for the gimmick.

        • RocketScientist

          Because we should be taking caliber recommendations from the guy who argued vehemently that .22 is a completely pointless round with no redeeming qualities whatsoever, and everyone who owns a .22 is a complete moron.

          • Frank Grimes


  • TechnoTriticale

    This risk has always struck me as being the predictable consequence of:
    a. starting from the .223/5.56 base case, and
    b. wanting to use existing STANAG mags.
    The mag sidewall ribs compel the shorter OAL, because the projectile ogive had to basically fit inside the .223 shoulder profile.

    This interchange hazard was a major reason I ruled out 300BLK when last contemplating an AR in a larger caliber. It’s also a significant confounder for those pondering various next-gen intermediate cartridges. No way is the mil going to adopt an otherwise attractive round with this obvious a kaboom potential.

    • Peter Marcus

      Note that this photo is not real 300 BLK ammunition. It is a short-loaded short bullet that no real ammunition company would ever make. The military has already adopted 300 BLK to replace some MP5s, MP7s, and HK-416s. They just order real ammunition (you know, like from an ammunition company) that cannot load in 5.56mm. They can even specify that on the ammunition contract.

      • TechnoTriticale

        re: Note that this photo is not real 300 BLK ammunition. It is a
        short-loaded short bullet that no real ammunition company would ever make.

        Thanks for that clarification. Just how much a tap on the FA would suffice to chamber a spec/crimped 300BLK in a 5.56 chamber?

        re: The military has already adopted 300 BLK to replace some…

        Sorry to hear that. It may not end well. There is an endless list of possible interchange hazards in small arms, some already mentioned elsewhere in the comments. Some are are just largely theoretical (like .357 in .38LC). Keep in mind that what made this TFB article worth posting is not a theoretical hazard. 300BLK kaboom reports are actually happening at a disturbing rate.

    • CommonSense23

      You realize the military is why the 300BLK exist right. The military has had the round for years now.

  • andrey kireev

    Here’s a reason why some people should just keep their platforms separated. If you can’t keep track of your rifles and ammo, then get an AR and an AK (or a Mutant or similar rifle)… 7.62×39 can do everything that .300BLK can, except rifle ergonomics (see Mutant)

  • Avery

    So, what you’re showing me, is that the problem is that we just need to give the .300 Blackout projectile flanged skirts and then it just becomes a squeeze bore? :v

    j/k it does explain why this has been happening with some frequency.

  • Christian Hedegaard-Schou

    Just FYI, this photo is using a bullet that’s intentionally seated deeper than normal, or a much shorter-than-normal COAL (or both).

    Typically, the bullet in loaded 300BLK will prevent a chambering like this due to a longer COAL.

    However, if the case/bullet isn’t crimped, then the bullet can be seated deeper by the bolt closing home, but not always or automatically.

    The above picture is NOT typical of .300BLK in a .223/5.56 chamber.

    • McThag

      Quite a few kabooms have happened because the bolt was able to shove the bullet down into the case. It’d look like the picture just before the boom.

      • Christian Hedegaard-Schou

        I’m pretty sure I said exactly that.

    • 🐒👊

      The short ogive bullets( originally designed for .308 win) can be pushed back into the case when cambering . This 110gr Hornady bullet in the picture is a great example. And there are many pictures to prove just that.

      • Christian Hedegaard-Schou

        That’s exactly what I just said.

        • 🐒👊

          And that is SPECIFICALLY what you just said SPECIFICALLY with specific references to actual things in this universe.

    • Cal S.

      Never underestimate the power of a forward assist in the hands of a newb.

      • DGR

        We need a new Acronym then: SPORK!
        Slap. Pull. Observe. Release. Tap. Kaboom!

  • Hoplopfheil

    I do want to make a 300 BLK build using some Strike Industries red parts and call it the “Redout.”

    But the risk of introducing 300 BLK anywhere near my existing rifles gives me pause.

    What’s a good alternative caliber? Or do I just say screw it and make another 5.56?

    • andrey kireev

      just get an AK !

      • Hoplopfheil

        I do want an AK, but I also want to build a fun AR. Is making a good 7.62×39 AR really doable?

        • Jeffrey

          I have one and it shoots like a really loud sewing machine

        • JustAHologram

          The trickiest part would be finding a lower that uses AK mags I’d you don’t want the use the proprietary 7.62x39mm AR mags. Otherwise the parts are out there.

        • Big Daddy

          I have 2 KS47 pistols from PSA, they work well and are fun to shoot. I also have a .300BO pistol and it too is fun. I keep all the 300BO stuff away from the the 5.56. I used a Noveske lower that had a tight magwell, so I only use the Lancer mags for it. It also has a Yankee hill upper receiver that says 300 BO on it and a port cover that says it. Also my only AR that has Magpul gray grip & sights, gray Shockwave and a Werne gray scope mount.

        • andrey kireev

          It’s Doable…. the problem you run into with straight up AR15 built in 7.62×39 is magazines… they don’t work particularly well…. There are several guns with AR15 ergos that use AK pattern magazines, that work a lot better.

    • James

      Color coded mags if you are mixing 5.56 and anything that fits in the mag well. Was a common shotgun issue mixing 20 and 12 ga. That is another recipe for disaster.

      • David N Johnson

        The 20 ga shell drops in and you can put a 12 behind it. Bad idea. You can also drop a 20 in a 16 ga barrel and fire it. It will blow gas back and knock your single shot gun open. Might hurt you, might not. I know a guy who did that, and he gave me the shotgun and all his shells. He thought his gun was defective, but I figured it out when I sorted through the shells.

      • iksnilol

        Yup, isn’t there some law about 20 gauge shells having to be yellow in the US?

        • AndyHasky

          I don’t think so, more of an unwritten rule I believe.

      • .45

        They are very obviously different sizes, I don’t understand how people get confused, regardless of color.

    • Tom Currie

      There’s always .458Socom

    • ExMachina1

      “But the risk of introducing 300 BLK anywhere near my existing rifles gives me pause.”

      General observation on this sentiment (not a personal attack): I don’t understand how we are all comfortable saying that guns don’t just get up and kill people, but then we somehow become uber-fearful when it comes to feeding them. Either we acknowledge that stupid accidents are inevitable and the only solution is to remove the hardware. Or we vow to simply pay attention, realize guns are no joke and treat them accordingly in every respect.

      • Hoplopfheil

        Totally agree, but I mostly worry about the people I take shooting with me.

        Not pointing guns at each other is pretty easy to understand. But the differences between two very similar rifle cartridges are more abstract.

  • valorius

    There’s a yikes waiting to happen.

  • Ark

    Good to know that a .223/5.56 AR will chamber and set off a .300 BLK cartridge if you’re inattentive enough to let it.

    I know we’re all operator af and would never let such a thing happen, but the magazines look the same and the rifles look the same and it seems like an easy enough mistake for even experienced person to make.

    • AndyHasky

      I use completely different mags for 300 blk. P-mags for 5.56, Lancers for 300. You could use colored electrical tape if you don’t want to diversify your mags. Also the 300 upper I have is significantly different then my uppers in 5.56.

  • Harry’s Holsters


  • Reazione Catena

    I know a couple of people who use only nickel plated brass in the .300blk; they say it is an immediate recognition factor…that said they only use brass in .223/5.56 …

    • JustAHologram

      Well if it keeps them safe…

  • AD

    Very informative photo. It makes me think perhaps the entire round should be redesigned or replaced to make this impossible, but obviously it’s a little late for that – and to be honest I’m not sure how it could be changed without losing the whole “all the same parts” advantage that was the reason for the cartridge’s existence in the first place.

    • Peter Marcus

      It is impossible if you use a 300 BLK bullet and not the shortest 308 bullet you can find.

      • DIR911911 .

        tell that to the people whose guns went BOOM

        • Peter Marcus

          I would tell them to buy properly designed ammunition from a company like Barnes.

  • cjleete

    Yikes that is too easy. Anyone have success loading heavier bullets for 7.62×39?

  • 22winmag

    .300BLK seems awfully popular with newbies and millenials. Go figure.

  • Peter Marcus

    Yeah but that is not a 300 BLK bullet and/or is not loaded to the correct length. If you get proper ammunition with a 300 BLK bullet, it cannot chamber if the bullet is crimped or there is enough powder in it. In other words, this is only a potential problem with improperly-made ammunition.

  • J.W. Ramp

    Very nice Nick – thanks for posting this

  • C. Her

    Is there a .300 blackout round that is suitable for hunting that won’t get chambered in a .223 barrel on accident?

    • ostiariusalpha

      Like is mentioned here and there, a good, strong crimp can help a great deal to make a mis-chambering less likely; there is no fool-proof way to ensure it doesn’t happen though.

      • Peter Marcus

        There is. You just load the cartridge with a powder that makes the case full, such that the long-ogive crimped bullet has no chance of being pushed into the case.

        • ostiariusalpha

          That is depending on always getting your crimp and powder load absolutely correct, by definition that is not fool-proof.

    • Peter Marcus

      Yes. All the Barnes ammunition.