a 300bk in a 223 chamber by accident?

There have been several threads where that has happened.  I haven't ever seen it happen in person, but I've read about it and seen pics where they claim it was the cause.

Yes.




http://www.thefirearmblog.com/blog/2015/01/13/psa-always-known-caliber-300-blk-5-56-kaboom/

We had a guy bring in a rifle that he claims that is what happened to him. I will see the rifle tomorrow at work and try to post some pictures if he ok's it. From what I have seen of the pics so far it was a complete fuck up. Broken bolt, lower receiver was cracked wide open on left side, upper was messed up as well. More when I have the gun in hand.

My turkey hunting partner blew up his 16 inch 5.56 by doing just that.  It destroyed the upper but the lower survived by some miracle.  Bruised thumbs and minor lacerations on the forearm so he was really lucky.  Good lesson learned.  You can do the same thing with the .338 Spectre and .30 HRT in the 6.8 SPC.  If his accident had not happened I would never have checked the combinations I mentioned.

Mine are all 5.56 for this reason.
I assume I could mark each one well and hope my memory stays perfect over the years but anyone else that used my guns now or in the future could be in danger if I had multi calibers. I have sons and grandsons who will someday be in charge when I am old or dead. .300 or 6.8 may have some SLIGHT advantage in some aspect but they neither one make any real improvement in making a hole in flesh over the 5.56. It is fun to have something different and new but the other calibers are just that ''something different'', If I want something that makes an actual difference I will go .308.
It is just far to easy to kill someone fooling around in calibers not made for an AR15 in an AR15. I would bet the majority of .300 and 6.8's are even marked 5.56 on the receiver which is just plain dumb. The ones someone actually took the care to be marked ''multi'' are still a ? to anyone picking up the gun.

The 5.56 marking on a .300 is a non issue.  Go ahead and try to chamber a 5.56 in a .300

.300 marking on a 5.56 would end up having a bad time.

That said, I stole someone else's idea on hear and used the dots on my 3rd gen PMags to mark dedicated 300 mags since adopting the caliber.

I wasn't in to the caliber either until I became interested in short barrels.. Now I find myself considering a complete switch over to 300.

I mark the clips and keep the ammo separated and labeled







and yes, that is a heavily armed RV

Never had it happen to me personally, but I spoke to a man when I was down in Texas who had this happen.  I use Lancer mags with my .300 and aluminum mags with my 5.56, so I have both visual and tactile cues that something isn't right before disaster occurs.