Will a .300 Blackout chamber (and fire) in a 5.56 rifle?

[TXAZ]

Was reading on another forum where someone claimed someone at their gun range had chambered and fired a .300 Blackout round in a 5.56 chamber.
I realize the back end of the cartridges are pretty close in diameter but that seems crazy.

[Jim Watson]

Yes. It is on the SAAMI list of DON'T DO THIS.
I have seen pictures of cross sectioned chambers, the .30 bullet rests against the shoulder of the 5.56 chamber, supports it against firing pin impact, and it will go off.
You won't like it, though.

[Sevens]

Haha, the back end of the cartridges BETTER be close, many folks form their .300 BO brass from .223. The whole idea is an AR with exact same bolt face and exact same magazines, that's a huge part of the success of the round.

[MC 1911]

I've seen it done once with ugly results.

[bedlamite]

[TXAZ]

Great pic bedlamite. Thanks and wow.

[gwpercle]

Thanks bedlam.....That's Scary.
I hope Bubba keeps his ammo segregated , looks like a lawsuit waiting to happen !

[Sharkbite]

With proper (in spec) ammo, the Blackout bullet runs into the shoulder of a 223 chamber BEFORE the case allows the bolt to close.

I make sure to put a good crimp on my 300blk loads to prevent bullet set back on chambering. None of my ammo will chamber in a 223 and allow the bolt to lock up.

[Sevens]

I may be mean spirited but I prefer that stupid people pay a stupid penalty.

If a guy can't be aware enough to put the proper ammo in, he's going to be made aware that he is careless in some manner or he'll continue his carelessness and recklessness ever onward and upward.

[Drm50]

Back when I had a shop there was a kid right out of Hi school that inherited a
very nice 300 H&H mag. He kept coming in a looking at the ammo but never
bought any. He never had fired the rifle and wouldn't sell it. I respect him for
that it came from a deceased uncle. One day he showed up with a box of Win
300H&H he picked up cheap at a flea market. They were hand loads of a wildcat
35 cal on Shortened 300 H&H case. They would chamber about same way as
300BO in 5.56/223 chamber. Kid was smart enough to see bullets were to "fat"
I never did pry that rifle out of him, he finally broke down and bought a box of
ammo. I believe that combo would have resulted in trip to ER at the minimum.

[armednfree]

That does not make for a happy day.

[T. O'Heir]

A .300 Blackout round in a 5.56 chamber will cause a seriously catastrophic failure. .308" is much, much bigger than .224". At about 50,000 PSI.

[GarandTd]

Sevens, I agree.

Sometimes, most actually, I believe government and the legal community do too much to protect not so smart people from themselves and punish everyone else for their mental shortcomings. Survival of the fittest. It's nature's way.

[jonnyc]

I have mt Blackout rifle marked BLK on the mag well, and all the mags I use in it marked as well. In addition, I only use 20 round mags in the 300 BLK and 30 rounders in the 5.56s. I'm planning that these efforts will reduce my "stupidity level".

[ed308]

Just search the web for 300 BO kabooms. Bad things happen when the trigger is pulled with a 300 BO round in a 5.56.

[marine6680]

When 300blk was starting to get more popular afew years ago, that started a rash of kabooms.

The problem usually stemed from people preloading magazines before going to the range, and taking both a 5.56 and 300 rifle to the range with them... Or they otherwise had magazines loaded with both calibers sitting on the bench. Heard of a few that loaded a single 300 into a magazine of 5.56 due to inattention, and a few of less experienced shooters being told to load the rifle and grabbing the wrong mag.

Even seen a few posts of people claiming to have found a single 300 in a factory box of ammo, where people speculated someone had attempted to sabotage the box to cause someone to blow up their rifle. Maybe or maybe not true.


Now it is recommended that you color code 300 mags in an obvious manner, or better yet, don't bring rifles to the range in both 5.56 and 300 on the same day... You still want coded mags either way, and never break the code for any reason.

Magpul makes dedicated and marked 300 mags. They are a bit different internally than 5.56 mags as well. Supposed to work better with heavy grain subsonic ammo, as it would want to bind up in the 5.56 mag. The marking isn't something that jumps out at you unfortunately... It just molded in, not a contrasting color.


An interesting thing that sees to happen a lot in these kabooms... Is that the 30cal bullet often actually exits the barrel. Extruded like playdough into a thin several inch long deformed slug.

The rifle itself usually blows out the magazine, bends and/or cracks at the mag well and the upper splits near the ejection port.

Definitely a dangerous situation one needs to take great care to avoid.

[ed308]

An interesting thing that sees to happen a lot in these kabooms... Is that the 30cal bullet often actually exits the barrel. Extruded like playdough into a thin several inch long deformed slug.

Or doesn't leave the barrel....

https://s14544.pcdn.co/wp-content/up...5.56-rifle.jpg

[marine6680]

Yeah not every time.

It's down to what type of bullet it is. The hollow point or open tip type can, FMJ won't.

[berettaprofessor]

Magpul makes dedicated and marked 300 mags. They are a bit different internally than 5.56 mags as well. Supposed to work better with heavy grain subsonic ammo, as it would want to bind up in the 5.56 mag. The marking isn't something that jumps out at you unfortunately... It just molded in, not a contrasting color.

You can, however, buy uncolored/sandstone base plates for Gen3 Magpul mags and color them to your liking using polymer fabric dyes. I've done it for my 300BO mags. All have yellow bases.

[Mobuck]

I doubt a heavier bullet loaded to magazine length will allow the bolt to close.

[marine6680]

Mobuck... Do you mean in a 5.56 chamber?

[Ed4032]

Yes it will but only for one round. ������

[Mobuck]

Mobuck... Do you mean in a 5.56 chamber?

yes

I suppose I could stumble downstairs and try it.

[reynolds357]

Join Date: July 28, 2007
Location: Central Ohio
Posts: 11,227
I may be mean spirited but I prefer that stupid people pay a stupid penalty.

If a guy can't be aware enough to put the proper ammo in, he's going to be made aware that he is careless in some manner or he'll continue his carelessn

Yeah, everyone should blow their hand or half their face off to learn a good lesson. Mean spirited would be too nice of a term to describe you.

[GarandTd]

A 300 blackout round and a 223 round are pretty easy to tell apart. If you don't notice the difference while loading maybe firearms aren't the right hobby. We're not talking about pouring OJ in your breakfast cereal. This is a little more than an embarrassing mistake.

We as gun handlers/enthusiasts owe it to ourselves and everyone around us to be be more careful. There is no excuse.