KABOOM: 300BLK fired in a 5.56 FS2000

I guess the good part about 300blk is that it uses the same bolt and magazines as a .223/5.56, but the bad part is that is uses the same bolt and magazines as .223/5.56. This happened 12/17/2013 when I was at the range with a buddy doing some recreational shooting.

Pictured is is my friend’s .223 FS2000 that he accidentally shot a 300blk round through. This was his first time shooting the gun. He is an avid reloader and a big fan of both 5.56 and 300blk, so one could see how a round or a magazine may have gotten swapped by accident. When he pulled the trigger a very strange noise could be heard and smoke started billowing out of the gun’s trap door behind the picatinny rail and the bolt seized. The case blew out into the bolt face so hard that we were not able to remove it.



I was wondering to myself how the round even chambered and it is quite simple. Reloaders make 300blk by cutting the neck off of a .223 case and resizing/necking it down to accept a .30 caliber bullet. The projectile used was a very short and lightweight bullet (125 or 130 grain) with no jacket on the rear. I assume that this allowed the bullet to fit in the chamber, and when it was fired I imagine the projectile expanded to be three inches or so.

When a bullpup catastrophically fails, it can be quite terrible because the action is right next to the shooter’s face. The fact that the gun did not explode and injure my friend I believe is a testament to FN’s quality of materials, especially their legendary cold hammer forged barrels. We inspected the gun and could find nothing out of the ordinary:




However the firearm no longer functions as the extracted rounds will not properly enter the ejection chute that is parallel to the barrel.

So 300blk fans, be careful and make sure this doesn’t happen to you. It could result in a broken gun, or serious injury.

Alex C.

Alex is a Senior Writer for The Firearm Blog and Director of TFBTV.


  • Steve (TFB Editor)

    Just last week a friend of my brother was out hunting. His hunting partner handed him a cartridge and he loaded it without looking and then fired it. The cartridge was incorrect but chambered and fired, blowing the rifle action apart. He had to be hospitalized.

    I am obsessed with checking the cartridges I load in my range bag or in my magazines.

    • TangledThorns

      I typically go to the range alone or with the wife so obviously all the ammo we have with us is mine. After reading your post I should be weary on what ammo goes where on that rare occasion I go shooting with friends.

    • noob

      sorry to hear about your friend’s incident. I hope that he makes a full recovery soon.

  • eric

    damn that was close

  • Fred

    There’s no way a 300BLK would allow the action to close in a .223 chamber. I’d be more worried about the gun being able to fire that far out of battery.

    • Ken

      He said it was a very short and lightweight bullet.

    • mefornow

      I agree. And there’s no way a .30 was swagged down to .223 without damage. I bet the more likely cause is a double charge. The action probably slammed open while still under pressure, causing that blownout/fireformed neck.

      /then again, anything’s possible.

      • You cannot really double charge a .223. The case is loaded to capacity. He also runs a dillon 650 w/ load senor.

        • sprky777

          Isn’t it possible that the light bullet was pressed back into the shell and the powder burned around it? After it was ejected, the bullet may have been in the casing or fell out onto the ground since the neck was expanded. I don’t think it would be possible to extrude a .30 down a .223 bore without bulging.

    • sprky777

      Isn’t it possible that the light bullet was pressed back into the shell and the powder burned around it? After it was ejected, the bullet may have been in the casing or fell out onto the ground since the neck was expanded.

  • M.M.D.C.

    Was it a subsonic round?

    • Giolli Joker

      Unlikely… the article says lightweight bullet, a subsonic round usually carries up to 220grs bullets, that by being longer would probably leave the bolt stuck before it can close the action… preventing such an incident.

  • Fred

    Have you thought about adding a newsletter to the website? I think a lot of people would be interested in signing up for one, you should add that feature and let people know that you have a newsletter!

  • Ed

    Well that’s what happens when you have a none modular FS 2000. Always check your ammo before loading another good tip.

    • Anonymoose

      It would be awesome if they made .300 Blackout barrels for the FS2000, or just moar FS2000 accessories in general…

  • hkryan

    A very good lesson learned. So far my biggest blunder has been trying to load 20 gauge in a 12 gauge auto, enough of an eye opener that I now keep everything clearly labeled.

  • Vhyrus

    Good thing it wasn’t a Tavor. I hear you get kneecapped by Mossad if you break one of them.

    • You get a laugh from me on that one! I’m always curious but always thankful that my Tavor has never had a catastrophic failure.

  • This is an all to easy thing to do and any shooter having both calibers in the same platform should use a reasonable amount of caution and separate the gun and it’s ammo from the other caliber.
    I was at Gunsite last week and we happened to be shooting AR’s in 300 Blackout and 5.56 in the shoot house. All the rifles were suppressed with 9 inch barrels for the most part. John Hollister the product manager at AAC was very adamant about separating the 5.56 ammo and guns a good distance from the 300 Blackout ammo and rifles. They’ve seen this mistake happen with worse results.
    It can happen to anybody and getting into the habit of checking before loading is just good range practice.

    • HSR47

      That’s pretty much my policy on 20/12 gauge shotgun ammunition: I *never* have both calibers of ammunition out when I’m shooting 12 gauge shotguns.

  • franklin

    Is the OAL length of .300blk short enough compared to 5.56 to allow the bolt to fully close on the round?

    • Anonymoose

      I wouldn’t think so.

  • Tuulos

    Luckily nothing happened to the shooter. I’d think easiest way to separate the ammo in these cases is to use different magazines for different calibers, for example E-mags for .223 and GI-mags for .300BLK with caliber written on the side of the mags might already go a long way to prevent accidents like this.

    • N4choLibre

      Or, just wrap the different magazines in different coloured tape.

      • I do this with electrical tape. Cheap and effective!

      • Tuulos

        I just prefer to keep it very safe when it comes to firearms. And since I rarely shoot alone but have friends and family with me it’s easier for everyone if there are clear differences and markings. And I’d probably forgot which colour was which and would have to check constantly.

        • N4choLibre

          For sure. Or you could write .300 or .223 on the tape. But whatever is easiest. 🙂

        • Cymond

          As I build my 300 blackout, I plan to use olive drab furniture and olive drab magazines so that it’s clear which mags go in which rifle. The trick is just being very careful to make sure that the cartridges don’t get in the wrong mags.

  • Blake

    I suppose the price to pay for magazine compatibility is mandatory increased awareness…

    • noob

      that’s the reason why 9mm broomhandle mauser pistols have a big red “9” on the grip.

  • LRB

    I keep hearing of these mixups and I have to say I am somewhat baffled. First, I reload, 5.56, and 300BLK and numerous other pistol and rifle calibers. So how in the world are people confusing 5.56 and 300BLK? 300BLK is a shorter case, with a bigger bullet. Even if the bullet loaded was a 110 grain VMAX there still is a considerable appearance difference between the two cartrdiges. The only place they look similar is viewed side by side from the rear and looking only at the case head. Are people just grab-bagging their ammo like halloween candy? I just dont get it, I really dont and no sarcasm. 8 years in the Army and shooting since the age of 4 I always learned of kabooms from improper ammo identification. Growing up there was a horrific accident involving a 12 gauge shell fired from the single shot 20 gauge at the range we would go to. I never forgot that. But I am seriously perplexed how this happens.

  • ChristianC

    I was having a lot of trouble one range day with my AK47 firing Wolf ammo. The cases were getting stuck requiring a cleaning rod down the barrel. The damn thing also wouldnt go all the way into battery 20% of the time. Turns out a girl that loaded my mags was mixing in 5.56 Wolf. Im an avid shooter, so when I tell you that I could BARELY tell the difference between steel case 5.56 and 7.62 when they are mixed together loose on the table, dont scoff too much. Took 3-4 hours of shooting to actually figure out the problem. So here it is: 5.56 will load into a 7.62 magazine AND FIRE. It will not extract of course. Safe shooting everyone.

    • Vhyrus

      Where can I get a magazine loading girl?

  • Martin M

    This is outright carelessness, and I have no pity for it. Far too often the excuse is made that the rounds look pretty similar, and that make the mistake understandable. It’s not acceptable. Firearms are dangerous tools and should be handled as such. Would you put unleaded in a diesel? A PCI-E card in a PCI slot? Salt instead of sugar? They all look pretty similar too. Carelessness is not an excuse!

    • noob

      there are a lot of careless people. I just try to stay away from them while they’re endangering themselves and others by not looking at or checking things.

      • mike

        I think the a mark of a well-engineered system (whether a rifle or a space shuttle) is the absence of single points of failure which can cause a catastrophic failure and injury or death. By that criteria, this is a terrible system. One tiny moment of carelessness can blow the gun up?

        (though it can be fun to feel smarter than those foolish, careless folks.)

        • Shakey

          The entire point of 300BLK, the reason why it exists, is to be compatible with existing AR parts.

  • Anonymoose

    One of my friends almost put .308 in his Mosin once, but we stopped him in time.

  • Tinkerer

    You know, I keep reading Internet gunsmiths saying that “When a bullpup catastrophically fails, it can be quite terrible because the action is right next to the shooter’s face”, or something of the sort. Can somebody -ANYBODY- PLEASE provide some documented and verifiable case of such an occurrence? Because the scientist in me is damn TIRED of hearing that belief being repeated over and over, and not find any hard data that backs it -because I haven’t found any data; my Google Fu is indeed strong, yet nothing has come up.

    • Christopher

      I can’t find anything either, its nothing more then BS made up by Bullpup haters in favor of what ever conventional rifle they fanboy over.

    • optac

      Hmm, like a bigfoot sightings. Lots of people seem to think they know someone who has seen the monster.

      I also consider myself to be a Jedi master of the internet search puzzle, but I was able to find one (dissapointing) bullpup failure of a Bushmaster M17s. To me it looks more like the stock (which acts as the buffer stop) failed rather than a true kaboom. Opinions welcome.

      The only backgroud info provided was –> “Supposedly, the cold made the plastic brittle, and combined with whatever ammo they were shooting, the rifle KBd”

      • optac

        Opps, New poster on this site. Not sure why I got the triple ditto on my 1 picture. Mods please delete the duplicates.

      • Laserbait

        That was a common failure mode for the Gen1 M17, no KB was needed to have it fail like that. 🙂

    • Well, here is an example. What if this guy had this rifle in a bullpup stock kit:
      Mind you I am grasping at straws here, haha.
      Luckily we do not hear about kabooms much at all, even in common rifles, but conventional rifles are more plentiful than bullpups by, who knows, 50 to 1, maybe 100 to 1? A shooter is not safe from any catastrophic rifle failure, much less one in a bullpup where your face is right next to gound zero. I gaurantee you that if I took some IMR 4198 and ground it up with a mortar and pestle then loaded a case with 50% over maximum and shot it in any bullpup, it would be a grenade.
      If any bullpup manufacturer is gutsy enough to send me one to try this, I will be more than happy to test it with some sort of trigger actuating mechanism and take plenty of photos and videos. Somehow I really doubt anyone will take me up on this!

    • Mike

      Tinkerer, I understand your desire for data – or even a documented example. But isn’t it just common sense that a kaboom under your cheek is worse than one 6-8″ forward and below your face?

  • Guest

    Swap the word ‘accident’ with ‘negligence’

  • Sulaco

    Was at the range when a 9mm was loaded from a loaded magazine into a .40 Glock. Fired without damage except splitting of the case and pushing the primer out.. Can happen to pistols also, in fact IMHO more likely then a rifle.
    Martin M where did you do duty with SAC? Grand Forks 75-75 here.

    • Martin M

      Huzzah for the SunFlake! I was at Minot.

  • Freebug

    How the heck is this even possible? The 300BLK will not chamber into a 5.56 one. I know, because its happened to me.

    • sprky777

      Is it possible for the bullet to be jammed back into the case when it hits the smaller chamber?

  • Brandon

    I have a Glock 20 and a .40SW conversion barrel; so when I go to the range I always double check which barrel is in and which caliber is loaded. I would have thought .300BLK would be easier to distinguish from 5.56 as opposed to 10mm from .40 :/