PSA: Always Known Your Caliber (300 BLK in 5.56 KaBoom)

Originally posted on Reddit, one very fortunate shooter survived a catastrophic failure by detonating a 300 BLK in 5.56 firearm. 300 BLK has many advantages, but can have one real disadvantage. It can chamber in a 5.56 firearm under certain circumstances.

Quote from Reddit:

Murphy’s Law struck hard this past weekend where somehow some way a 300BLK round made it from the box on one side of the table, into the mag of another AR on the other table. No one was hurt due to proper PPE. The lower survived with just a broken bolt catch and the upper, as one would expect, is a complete loss.

Always make sure to know your caliber.

Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • Kurt

    A link to the reddit thread would be nice….

  • Kivaari

    That is amazing. It appears to be an 80% lower, that worked.

    • Christian Hedegaard-Schou

      What’s “amazing” about that? There’s nothing special about an AR lower. There are no special or critical tolerances that an AR lower must maintain in order to function. Plenty of people have made very crude lowers out of blocks bolted together, wood, and even cutting boards that have functioned just fine.

      • Kivaari

        It’s amazing no one was hurt, and the lower was not ruined.

        • It’s rather unusual for a kaboom to kill a lower on ARs. The pressure tends to blow out the mag, which then vents the rest of the gas. On an AK or similar rifle with a rock-and-lock steel mag, you can get a lot more damage.

          I shudder to think about what this would have done to someone rolling with a bullpup, though… even if the upwards portion of the blast was contained (and most bullpups are designed for that), I’m guessing having that downwards venting would suck real bad.

  • Risky

    I still don’t see how you could ‘chamber’ a .300 blk in a 5.56 upper. Having to beat the foward assist with a rubber mallet should be your first indicator that something is amiss.

    • You can and on the range everyone takes safety precautions to make sure the 5.56 and 300 ammo is separated.

    • MadKaw69

      I’ve seen it done. The bullet itself gets pushed back into the case and the case chambers. Drops right in. Saw the aftermath. Blew out and split the PMAG (PMAG still worked afterwards). Split the upper (not as bad as above). Round went all the way through the barrel (looked like a pencil) and into the sand barrel! No one was hurt, luckily.

    • spotr

      Like this!

      Here is a quick photo shop based on Christian’s description above.

      Accidental loading of a .300 into a .223, plus a weak crimp, or a projectile with no cannelure (groove around the cylinder of a bullet), or a hard rap on the forward assist.

  • Christian Hedegaard-Schou

    If you are a home reloader, one thing you can do to help partially avoid/mitigate this problem is to reload using cannelured projectiles and giving your brass a good crimp at the cannelure.

    300blk will NOT normally chamber in a .223/5.56 chamber. What happens is that if you have a loose projectile, or a non-crimped projectile, when you chamber .300BLK in a .223/5.56 is that the bullet gets pushed back into the cartridge case enough to seat the cartridge.

    If you use cartridges with a good crimp on a cannelure, you should be able to almost completely avoid this.

    Of course, it goes without saying (look I’m saying it anyways), pay the F attention to what you’re doing from the beginning anyways!!!

    • spotr

      Thanks for the explanation of how it could chamber a larger diameter projectile in a smaller hole.
      This didn’t make sense to me at first until I visualized your explanation of the projectile being pushed back into the case. It would still need enough force to crimp the neck of the .300 blk case so that it fit into the neck chamber size of a .223, but at least now I can see how it is possible. I wonder if the forward assist was used?

      The end result would be a crimped neck case, with an internal plug (the projectile) and a powder charge behind that. If the bolt locked closed then the described device would now be a bomb. Kaboom!

      • Rybred

        Actually the case doesn’t get crimped while clambering at all. In order to handle a larger, longer projectile .300blk cases are much shorter than .223
        In fact you can chamber the .300blk brass in a .223 chamber just fine. The only thing preventing it from going in is the bullet itself. Take a bolt carrier slamming home on a cartridge without a strong crimp (it only needs to be pushed back a 1/4 inch or so) and you can see how it’s not difficult to make this happen.

        • spotr

          Right! I created a photo-shop picture (below in another comment) which shows that no case crimping is needed, exactly as you have stated. “Seeing is believing”!

        • Giolli Joker

          There might even be a combination of light (short) bullet and forward assist habit to help in the disaster…

    • NDS

      I always crimp ALL 300blk loads to a varying degree, for two reasons:
      1. Don’t want to blow up as seen here
      2. Improves accuracy, especially on the light 110gr-125gr stuff, by keeping more consistent velocities. Lots of trial and error as to how much crimp each projectile likes though.

      A super easy way to make sure this NEVER is an issue: All my 5.56 mags are black. All my 300blk mags are FDE. They are labeled and numbered.

  • Esh325

    This is off topic, but why haven’t they ever came up with a rifle that automatically closes the bolt when a magazine is inserted opposed to having to push a bolt release?

  • TechnoTriticale

    Which other popular alternative AR-15 cartridges have this same risk?

    I know the 6.8 SPC doesn’t (won’t fit in 5.56 chamber or 5.56 bolt face), which was a factor back when I was considering something more suitable for deer than 5.56.

    The problem, of course, has likely been around as long as centerfire has; .38 Super in .38 ACP (or 9mm Largo) for example.

  • me ohmy

    damn shame.. looks completely bas-ploded..hope their okay

  • whskee

    Ruh-roh…ghost gun got ghosted! Glad there was no injury to the shooter other than the hit to the wallet!

  • patrickiv

    I’m glad we got to learn from this without anyone dying.

  • highhammer

    yikes. once me and friend brought out a new shooter. had a verity of arms out at the range. braked for lunch. the excited new guy made it back to the range before us and we found him loading 7.62×39 in to a 5.56 pmag. i was surprised he was able to fit it in there. thankfully we were able to laugh at his expense instead of making a call to 911.