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.

        • Old Vet

          Good looking dog.

          • mbrd

            tactical dog?

        • Sam Adams

          You’re killing me! We should go out for a tactical beer sometime.

    • 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.

      • gregge

        What about old ones? I’d like to see an AR in .303 Savage, just because.

    • 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?

      • Jim Smith

        Administrative controls are not effective. People get complacent, lazy or just careless. I do some work in industrial safety, and we go to mechanical controls. Take the human decision making out of the equation. In this instance, mags would have to be made incompatible with weapons not chambered for that caliber.

    • Rooftop Voter

      You aren’t Alfred Schrader under a different name by chance?

      • Martin Grønsdal

        Who is that man?

  • 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!

          • Jim Smith

            And don’t forget to say “Just hold my beer and watch this.”

        • 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.

          • Andrew Foss

            TL;DR: Not with ordinary tools.

            Tannerite requires >1,250 peet per second. The government mandates that lawnmower engine RPMs go no faster than 2k RPM and the blade tip covers no more than 19k feet per minute. This means that even an optimal hit by the leading edge of a lawnmower blade would be 316.66_ feet per second: Not enough to activate the Tannerite. Now if you bypass the engine governor and stick a different pully on the engine spline, you could conceivably get close. But that’s less fun than shooting the Tannerite and then the lawnmower.

        • law-abiding-citizen

          Just bought a zero-turn mower. Did LOTS of research. Lowest specs I found for blade speed (at the tip, of course) was about 13,000 feet per second, up to about 18,500. I’m thinking yes.

          • Charles Applegate

            Well, hell – what are you doing tomorrow?

          • ytubepuppy .

            Uh, 13,000 fps is Mach 11.5. 18,500 fps is Mach 16.4. I seriously doubt it, since either speed would melt the blade.

          • AlDeLarge

            … or a 20″ blade at 149,000 & 212,000 rpm

      • III%

        ! a f**

        • III%

          ah it blew my leg off!! lol what a f**ktard.

      • Tonewall Jaxon


    • 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…

      • cawpin

        With certain lightweight bullets, it will go into battery without force

  • 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.

        • Jim Slade
          • George Dean

            Sorry Jim, but you’re warping my brain. Are you not discussing a 16 1/2″ tire on a 16″ rim then depicting a 16″ tire on a 16 1/2″ rim? This is the kind of stuff I encounter at home, and then she-whom-I-adore says “Oh, you know what I meant” when I call her on it, while my brain is bouncing off the wall. 😉

        • iksnilol

          Thanks for the info. Hot dang, that doesn’t sound good.

        • Sam Ruez

          you can take your eye out with that!!!!

  • 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!

    • mb

      Nothing to be proud of, someone was watching over you. Hope you are never unfortunate to have that happen again. I thought of buying a 300AAC upper for my rifle until I realized the possibility of a 300BO getting in 5.56/223 chamber. Solved it for me, AK47M was solution.

      • USMC2090

        Trust me…I was not proud at all…I was absolutely grateful that nothing happened to me. I dont even own a 300blk, but we were at an open shoot and apparently someone left a 300blk mag out on the table without marking it.

    • mbrd

      i’m having a really difficult time imagining a .308 bullet making it through a .224 bore at all, let alone “without ruining the barrel”… hk416 robustness notwithstanding.

  • 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!

        • Jim Smith

          SPORK? KFC will be filing suit for copyright infringement.

  • 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.

      • Vanns40

        I bought clear mags for my 300 and black for the 5.56.

    • 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.

      • Vanns40

        What in the world are you talking about? My guns are seething, snarling beasts. I spent a fortune on handcuffs and a reinforced steel bar to restrain them just so friends and neighbors would be safe and my homeowners insurance wouldn’t be voided. Oh wait, maybe that was an ex…….oops, disregard.

    • AlDeLarge

      If you’re building a pistol or SBR and/or going to use a silencer, 300 BLK is the way to go. If not, not-so-much.

      • 80sDweeb

        Good friend of mine LOVES hunting deer with 300 BLK out to 200 yards. Says it’s like a 30-30, but with less powder, less recoil, and less noise. Likes his 6.5 Grendel for hunting out to 350-400 yards.

        • AlDeLarge

          That’s true, but my point is that if you’re going to do one of the things I listed, 300 BLK is the obvious choice. Anything else will be situational. It fits your friend’s situation well. If the OP is just building an AR for the hell of it, there’s not much reason to bring it into the mix.

        • uisconfruzed

          This sounds like me, are we good friends?
          My avatar is an Alexander Arms 6.5 hard use bolt.

  • 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.

    • uisconfruzed

      Along with this 50 year old gun owner for over 35 years, and a 17 year spec op vet friend of mine.

  • 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.

  • 3==>~Sharia

    They should start making the ogive higher on the .300blk so it won’t seat in a .223 chamber then.

  • Rock Island Auction

    “Perhaps this image may help explain…”

    While many of us know the dangers of loading an improper cartridge, this still does not explain to the uninitiated WHAT they are seeing and WHY that is bad. As is, it’s just a nice photo that someone can use to explain the scenario shown.

  • John Wisch

    BOOM !

  • cawpin

    Are we just taking content without attribution now?

  • George Dean

    Mark your gear, mark your gear, then check to see if you’ve marked your gear! And don’t let anyone else use it without close supervision or proper orientation.

  • robotprom

    subsonic blackout won’t fit into a 5.56 chamber. Supersonic blackout will, if certain conditions are met. Like most careful people, I use certain mag brands only for 300 blackout, and certain mag brands only for 223/5.56. the 300 blackout mags also have a bright purple stripe painted around the lips.

  • Atlas

    When is TFB going to get over their crusade against 300 blackout? Did one of the writers get personally slighted by someone at AAC or something? Get over it already!


    I think there’s an award for doing this sort of thing. If I recall correctly it’s the Darwin Award.

  • Richard Lutz

    Some people don’t believe me when I tell them they can chamber a .300 AAC Blackout round in a 5.56mm/.223R chambered rifle but here is visual proof. Anybody who has both .300 ACC Blackout chambered rifles and 5.56mm/.223R chambered rifles has rocks in their head.

  • BeenThereDoneThat

    This clarifies the problem as I understood it! I was under the impression, the WRONG impression, that correctly loaded .300BLK would NOT chamber in a 5.56/.223 chamber and still allow the bolt to go into battery. While I do mark and store all my .300BLK ammo and mags completely separate from my 5.56/.223 gear, I am now going to be double diligent about this.

  • uisconfruzed

    I need to remove the pin & check both of my 5.56/Wylde rifles.
    I load the 300BLK’s long & crimp the bullet in place.

  • wjkuleck

    I’ve been at this gun fun for over sixty years now. You should know that there are many more mismatching opportunities than .300 Blackout in .223. Ponder all the Mauser-based cartridges (.470″ case head) from .243 Win up to .35 Whelan. For example, after WWII it was not unknown for a 7.92x57mm cartridge to find its way into a .30-’06 chamber, with less-than-salubrious results. There were other, even more destructive combinatorial opportunities, exhaustively documented in the gun magazines of the day.

    A fairly common “oops” in the days when Service Rifle competitors were transitioning from Garands to M14s and M14-type rifles was .308/7.62-MM in a .30-’06 Garand. The result was generally the fireforming of the .308 case into a “.30 Basic” straight-wall case and a clean miss on the target..

    I have 300 Blackout and .223 rifles, including an SR556TD with both .223 and .300 Blackout barrels. There’s a simple way to avoid a goof; use only distinctive magazines (Ruger prominently marks their .300 Blackout Mini-30 and SR556 mags) with your .300 ammunition. In addition to Ruger magazines, my .300 Blackout AR-type mags are exclusively Hexmags with orange followers and floorplate inserts. Is that so hard, really?

  • phil box

    yikes. don’t want to be anywhere it when she blows.

  • ronwf

    Perhaps a demonstration – safely, with the firearm fired using a remote device behind a blast shield – would get people to understand the danger.