Shooter fires 300 Black Out Through a 5.56MM


The old 300 blk out/ 223 mix up strikes again. B-ARFCOM user Plank_Spanker shared this story and picture about his very lucky but yet inattentive buddy.

So a buddy of mine was at a range doing some shooting a few weeks back.
Shooting both .223 Rem & .300 BO uppers on a single lower switching back & forth.

He somehow got a .300 BO round mixed in & extruded it halfway down the barrel of his .223 Aero upper.
At that point he didn’t realize what happened. Bolt jammed up couldn’t mortar it out.
He brought it to my place & I was able to get the bolt free & low & behold, a .300 BO casing was stuck in the bolt. Popped primer fell out too.

When the rifle jammed up, he said it made a “pop” & like cooked off the propellant with lots of smoke. I had read that same description in another thread here.
I didn’t have a rod to try & pound it out so he took it to his LGS & had it removed.
We are all amazed that it didn’t kaboom the rifle. The .300 was a subsonic round.
Smith said the barrel & bolt were fine

He wants to have a jeweler mount it on a chain as a reminder, I told him it would be a better reminder as a butt plug.

Be careful out there.


  1. What that guy was doing sounds like he was asking for trouble. I always try to arrange my life in a way that helps me avoid tripping over my own dick. It’s a good policy to follow.

  2. Wow. While reading, I assumed he was shooting a cast bullet which would probably make it easier to jam the BO in the chamber and easier to extrude. High-speed slugging of that barrel. No kaboom? Sturdy is a good word.

  3. I had that happen to me a few years ago. Took my 5.56 suppressed AR out back and did not notice the top round in the mag was a 300BLK subsonic. KABOOM!
    Fortunately minimal damage. It was a 208gr. BTHP round. The core shot all the way through the barrel leaving the case stuck in the chamber. The bolt extractor was damaged, the BCG was jammed halfway to the rear, the 10 round magazine was blown out and parts flew everywhere. The case had separated at the base leaving the rest in the chamber. After I was able to get the BCG retracted and separate the lower and remove the brass from the chamber, the only damage was the bolt and the magazine. I swapped another BCG from my other AR and the rifle functioned just fine. No barrel damage. The rifle shoot .5 MOA at 100 yards with 75gr. Hornady Match handloads.

  4. Am I the only one who uses labels on my mags to avoid this mixup?

    Faxon Firearms has color-coded “bands” for .300 BLK (red) and 5.56 (blue). They’re great.

  5. Guesty, I now use color coded or letters for magazines. Black for 300BLK, Desert Sand or Camo for .223/5.56, .458 SOCOM printed on those mags and the 350 Legend 10 round black mag is a straight not curved mag. The AR10 magazine won’t fit anything else. And a CMMG .22LR magazine that goes with the conversion kit on one AR. Only mag that fits that AR.

    Shawn, I did get a face full of gas from the blowback. Eye protection so nothing more than the smell of burnt powder.

  6. FWIW for those to whom this might happen:

    1. Well, some of us gunsmiths told people that this would happen when the .300 BO first started getting lots of press. We told you. We were pooh-pooh’ed as “crotchety old men who think everything invented since the .30-06 is shit.”

    Well, not ‘shit’ per se. But unnecessary? Yea, guilty as charged.

    2. Should this happen to a gun you own, take care to note the following: Not only is the bolt head compromised (and should never be fired again), but the barrel extension should be removed and a new one put on. Then a new bolt should be obtained and the chamber checked for headspace again.

  7. I know someone who has a 5.56 and a .300,he not only didn’t laugh when he saw clearly different/stamped mags for .300 but immediately got 20.He also noticed that dust covers sold with .300 on them,figured that he could paint on dust cover door(and did).This story started out not with a harsh lesson but wondering how to safely add another layer of,well……,safety and did,story so far has a happy ending.

  8. So, it isn’t often on a blog site that you get Paul Harvey’s “The Rest of the Story”…

    I am the guy that shot the 300bo subsonic thru my Aero upper.
    The picture above comparing the rounds is mine also.
    So, some corrections are needed.
    “ “So a buddy of mine was at a range doing some shooting a few weeks back.
    Shooting both .223 Rem & .300 BO uppers on a single lower switching back & forth. …”
    I was NOT switching back and forth between uppers. I never take both calibers out at the same time. I also separate my mags; PMags for the 5.56, and HexMags for the 300bo.
    The truth is that somehow, at home, I mixed a single round of .300bo subsonic into my 5.56 when I was filling my mags while getting ready for the range.
    “… he took it to his LGS & had it removed. ”
    I did not. I took the upper to my gunsmith. He and I removed the extruded bullet. He then thoroughly examined the upper, bcg, barrel, headspace, et al. He pronounced it good to go.

    Since my experience I have shot several hundred rounds thru it. As Accurate as it was before the caliber mix up; which is excellent. And I also use this upper with my CMMG 22lr conversion bolt and mags. Runs like a top w/ that also; and is very accurate w/ the 22lr.

    I relearned a very valuable lesson, one that my carpenter uncle had taught me beforehand.
    “Measure twice, cut once.” You Cannot Be Too Careful. I check and recheck everything now.
    And that’s how I roll.


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