300 blackout in a .223, opps


From what I understand, this is the result of putting a 300 blackout in to a .223. On our Twitter page, there has been a lot of talk about how it’s not possible. Tomorrow I will be running a test and recording it. I will take an empty brass from a 300 blackout, put a bullet in it and try to chamber it in a .223. I predict it will close, but not easy. I believe the bullet will be pushed in to the case allowing the bolt to lock.


Ouch… Talk about having a bad day.


I think the worst part is, that’s right in your face. I picked up these magBands from https://magbands.net/ a while back. I have way too many calibers that fit the same mags and look to close to each other.


Indeed. Even if you were uninjured you’d learn that lesson real quick!


How hard would it to be getting past the flinch.


I couldn’t imagine. Would it be unreasonable to assume your instincts would be to throw it immediately afterward? Nah, probably not unreasonable.


If that ever happened to me, I would need to purchase new underwear, and perhaps new pants. :thinking:


Holy crap! Where did you get that sample? Hope whoever this happened to is ok. That’s the kind of mistake you only make once.



Not really , I hear it happens to Glock owners at least several times a year.



From personal experience…

When 300Blk was brand new, and we were still confused what the difference was with the Whisper, I built a rifle to go play. Living in the People’s Republic of Khalifornistan made an SBR impossible (and pistols weren’t really very popular yet). I decided on a 14.5” pinned barrel and then built myself fun little Dissipator.

You can see my build HERE

Being a young idiot, I only built an upper, and was upper swapping at the range. I blasted off a few rounds of 300Blk, and and when I was satisfied I swapped back to my 14.5” 5.56 upper.

I shoved a fresh mag in the rifle, racked the bolt and squeezed the trigger… click!

I looked and saw that I wasn’t in battery. I smashed the foreword assist and nothing moved. I cleared my rifle, only to find that I had mistakenly loaded a 300Blk into my 5.56 upper.

From my measurements later I determined that I was just far enough out of battery for the FCG to lock back, but still be functional. When the hammer dropped, it would hit the bottom of the bolt carrier. It never touched the firing pin.

That being said… I did not shoot reloads or hand loads. I was only shooting factory new ammo. I’ve seen a lot of kaboom pics. I’ve even seen a round that didn’t kaboom and just forced a .30 cal pill into a .22 cal pipe, elongating the projectile into a 200gr .223, that got stuck mid barrel.

I don’t discount others experience. Just sharing my firsthand knowledge. I feel there are lots of variables that come into account when these possibilities come into play. We are playing with machines that have small differing tolerances. As those tolerances stack in opposite directions you may or may not get the same results.


Ps. If anyone cares. Here’s the link to my finished 14.5” Dissipator.




I sold that version. I wasn’t 100% satisfied with it.

I have a 13.7” Dissipator on the bench right now. Still using a rifle length handguard, but scrapping the front sight all together. This is an entirely new direction.


Not quite .300 in a .223, but when my father and I went to the range once, we brought my XD Mod.2 in .40, his P320 in 9mm, and his recently purchased Kongsberg-Colt authentic numbers matching German made WWII era 1911. After shooting our carry guns, we put a couple rounds through the Kongsberg, and satisfied, put it to the side to keep it somewhat pristine. At the end of the day, we decide to finish off our remaining .45, so we stuff a mag in, pull the trigger, and get a jam. Not sure what happened we pulled it apart and realized there was a case stuck in the chamber. Confused, and attributing it to the guns age, we pack it away and head home. Only later did we realize we sent a .40 through the .45, and both of us felt like we had angered the Norwegian gods of old. Lesson of the day? Keep your ammo separate when shooting, especially when it comes to historical pieces.


When I was in the police academy (MANY years ago) we had a guy mistakenly load 9mm into his S&W mod99. He dropped the slide on a full mag, and the entire cartridge spit out through the barrel. It barely dribbled out the barrel and rolled on the floor. His dumbass just sat there staring, with his hand in the air, waiting for a range instructor to unfuck his situation.


I look forward to seeing your new build