Never thought this would happen. I keep ammo cans separate. I don't shot both calibers at the same time. Not really sure how it happened but 2 300 blackout rounds found their way into a 5.56 dedicated Ammo can with about 800 rounds in it.

I was shooting with NVGs and loading out of the can in the dark. The rounds must have slipped by the feel test.

AR15 SBR
Factory 11.5" YHM upper (2011) maybe 4-5k rounds through it.
Factory RRA SBR lower (2008)
Aero Precision Pro Series BCG
YHM Phantom 5.56 suppressor

The Bullet made it about 6-7 inches down the barrel just short of the gas block.

Blew the Okay industry bottom plate out but mag stayed locked. Bulged out the bottom just a little. Still works.

The bull barrel didn't seem to bulge at all but the barrel extension cracked.
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Locking lugs on the bolt head are damaged so are the locking lugs in the barrel extension.

Extractor looks like a banana.

Bolt split.
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Upper receiver bulged. Broke the top off the bolt release. Bent the pin on the dust cover.
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I got 2 small pieces of brass the hit my foot and just broke the skin.

Thought I would share my fuck up.

Wowzer

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Salvagable parts

entire lower minus bolt release
muzzle device
suppressor
gas tube and block
handguard
charging handle

$400 mistake

Damn, that sucks. How did the lower fare?

Glad you are OK, tools can be replaced.

Mixing the two calibers up is what has kept me away from 300BO. It's a cool, useful cartridge...but to train with it at meaningful volume I can't shake the feeling that at some point Murphy's law would bite me in the ass.

Seems fine but I haven't fired it yet. Mag locks fine.

It happened on Friday. I rebuilt the gun last night.

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New stuff is a BCM upper
BA Hanson 12.3" barrel
Same Aero pro series BCG

Do not take this as me being an ass.

$400 in parts is easily 10-15 magazines. Have your mags loaded. I always hated waiting for people to load mags at rifle classes. Why waste time? If the mags were already loaded, you'd have been fine.

With that said, find someone to chop the barrel, would be a cool gun room art wall hanger.

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Very true. This wasn't a class. Just me shooting on some property I have and I wasn't running full mags. But all excuses. This was 100% preventable and my fault.

That sucks man. Sorry about the rifle, glad you're ok.

You at least caught a break no permanent injuries and no damage to the registered lower.

Thanks for sharing, its good to be reminded that safety never takes a break.

Thanks for having the balls to post that.  Most people just wouldn't.

And yes, .300 BO is pretty much the No 1 source of AR Kabooms now.  Your's is the second one I've heard about, in about 1 month.  A friend got to watch his friend Kaboom a $2K AR with .300 BO last month.  The irony in that one, is that dude didn't even own a .300 BO, and refused to believe it could have happened.  Until they beat out the 200 gr bullet and pulled out the shattered bolt to get to the brass.

The fundamental error everyone seems to make, is that they are completely conscious of every round they touch and insert into a magazine.  No, you're not.  Are you conscious of every footstep you took in the parking lot to your car?  Or every piece of pop-corn you eat?  No, you're not.  You think you are, because you don't remember that you forgot to notice something; or you would have noticed it.  It's a false self-check diagnostic.  Wildly more false than most people are willing to admit.  There's this perception that every single round is picked up, carefully inspected, headstamp confirmed, bullet shape confirmed, and carefully inserted.  Vs grabbing rounds and stuffing them in while thinking about somenthing else.  Or talking to someone else even - which is what really happens way more than people admit.

My problem with .300 BO  and my frustration as an engineer with it, isn't that it's unsafe itself.  A .300 BO round in a 300 BO gun is totally safe.  The furstration is it's an engineered system that in the wild, has the unsafe condition of being dissasterously unsafe in other guns and magazines that are often intermingled in usage and handling.  There's a reason .357 magnum is longer than .38 special, and it's not to hold more gunpowder.  Almost every other round is safety considered in a manner to minimize risk of mischambering and kaboom.  Except .300 BO, where some failure of a safety engineer thought it was a GOOD IDEA for this round to use the same bolt and magazines, even though it was possible to mischamber in a 5.56 chamber.  Whoever did that, should have their credentials pulled, if they ever even had any.  And shame on SAAMI for authorising this Kaboom maker.  They knew better, and caved to the pressure.  And now lots of AR's are Kabooming out there.  

The good news is the AR design is so safe, that usually the Kaboom is restricted to wrecking the rifle and only superficial human damage.... usually.

As an engineer who applyes safety, and indeed system safety consderations - I despise .300 BO.  It's not even that great of a round.  Grendel kicks it ass in every way; and won't kaboom your gun.  If you're running a supressed 8" Piston SBR; that's about the only place that .300 BO is even worth trying; considering it's basically a high-pressure straight wall cartridge, so those shine in short barrels.  Now that the arm-brace topic is a mess, why would you bother with a 16" 300 BO, where a bottle neck will beat it ballistically like a bad step-child?

$400 is a very small price to pay for this safety failure on my part. Gun could have been a total loss. I could have been seriously injured. One of my kids could have been shooting.

This seems to be a too common occurrence

300BLK claims another one. Glad you're OK OP. Have tore down an upper destroyed by a 300BLK before.

I don't understand why those who engineered it made it to where it would chamber in a .223/5.56 chamber. This is one reason why I never got into that cartridge.

.277WLV was going to be my answer to 300BLK. Only problem is/was factory ammo availability...

We're only human.

How did that happen if he doesn't own a .300? Was one mixed in with his ammunition he purchased? Or was this like a communal range day where people are sharing magazines to many different rifles?

Glad you're ok, OP.  Anyone else now see why Magpul is releasing clear mags?  Though that would not have helped OP, reloading in the dark.

Lazy, SAAMI authorized .300 Eldest Son on the basis that a shooter chambering a round in a bolt action would feel the bind and stop.  This has prevented .30-06 from blowing up .270 rifles for many years.  What they forgot to consider was that this cartridge would be primarily used in ARs, where we add H2 buffers instead of adjustable gas blocks, and are taught to beat on the forward assist if the bolt doesn't go home.

Apparently he has a pile of loosies, and decided to just run them.  I guess that must have included random from outside his own original supply route.  I don't know the guy, but I get loosies, at times - and will run them.  But when that happens, you bet your ass I inspect each one with a microscope, study the bullet, the brass, the primer, the neck - to discern if it's factory or a reload.  A 55 or 62 SC, a 77, etc.   I guess he did not.

You can't chamber a 30-06 into a .270 no matter how hard you try.  The neck dimensions won't fit.  Slamming it harder won't change that.

Glad you are ok.  That some.scary @#$$

Ah, yeah I'm always very careful about inspecting any ammunition I come across where I don't know the original source.

The whole .300 thing reminds me of this scene.

The 300 BO round does not chamber in a 5.56. The 30 cal bullet would stop short of fully entering the chamber so it does not seat properly. What occurs is the shooter fails to see/or know the bolt not fully closing and seating the round in the chamber like it would on a correctly sized round, before pulling the trigger. Thats why the ÓP's barrel bulged, the bullet was larger than the hole, and the explosion in the receiver caused the damage to the innards and bolt.

After building up a 300blk, I look at every round when I load mags. 300 and 5.56
When I go to the range, whichever caliber of the 2 I am firing, the other rifle and ammo can with loaded mags stays in the truck until I'm ready to fire that caliber.

The 300blk upper has a laser engraved dust cover stating "300blk"

A few friends say I'm too over cautious.  I tell them I'm not going to risk blowing up a gun because I was lax and could have easily prevented it.

Glad your okay OP.

Supers can - first chambering.

OST

I only had the one rifle out that night with only 1 ammo can and 3 mags. All loaded on site as I was shooting. 5-15 rounds at a time.

On a previous range trip or at the house cleaning up loose ammo I must have put 2 rounds of 300 in the can.

The 300 rounds were supers. 147 remanufactured training ammo I believe.

Damn!

That is one of my worst nightmares

300 danger noodle

Most supers, especially anything 125gr and under, will absolutely chamber in a .223/5.56. Some subs will chamber but the heavier bullets with fatter ogives won’t chamber.

There is no way for a .223/5.56 round to chamber in a 300blk barrel. The shoulder prevents it from fully chambering.

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Can you drop a 8.6 Blackout in s 6.5 Creedmore barrel?

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Glad you’re ok OP. The only time I’ve seen this happen was at a nvg shoot.

I don’t 300bo for this reason.

"The bull barrel didn't seem to bulge at all"
Readin's for rich folk

I have zero idea how people just let a 300 black out round just slip by them. Projectile is objectively fatter, case shorter, and the neck is just fatter too.

Dunno if you've noticed, but the back end of .223 & .300 ES are the same size.  Which end of the cartridge do you handle when loading?

This is why I don't allow .300 Eldest Son in the house, despite Fatalwishes leaving a nicely set up .300 in the safe.

No illumination. Wearing nvgs wasn't even looking at the ammo while I was loading because the nvgs weren't focused in close.

What's the story behind ".300 Eldest Son"?

Supers with a short ogive will chamber in a standard 5.56 chamber.

That’s why I’ve loaded pretty much only 220gr subs that won’t chamber in a 556 for training and plinking.

My supers for SD are in specific mags(300 pmags, 40rd pmags, and clear 20rd Lancers) marked with a red paint marker.

Impressive!

Good PSA OP. Glad you’re OK.

and this is why my dumb ass will never own 300bo.

https://en.m.wikipedia.org/wiki/Project_Eldest_Son#:~:text=Project%20Eldest%20Son%20

I have one 300blk I use with NV and a suppressor for varmint control. I keep the ammo and mags seperate. I also use only Lancer clear mags for the 300.

But now I'm thinking maybe I should use the clear mags in my 556 guns to make sure no danger noodles are loaded.