One really good way is to fire a .300 BLK in a 5.56mm rifle. Here’s a story of such an event, from a bystander who talked to the lucky (to escape serious injury) shooter, who was transformed in milliseconds from an AR owner to a former AR owner in possession of some scrap metal.
Once we determined the shooter was physically OK, I wanted to get out of their business, so I didn’t get any photos of the rifle, but I can describe the damage. In short, it was pretty much totaled. Perhaps the Magpul front hand guard, rear stock and trigger group can be salvaged. That’s about it.
The magazine blew up, along with spring and follower. And you can see what happened to the other rounds in the picture here. I *believe* the fact that he was using a polymer magazine may have saved the shooter from additional injury. The explosion clearly took the path of least resistance. Perhaps a metal magazine would have allowed more pressure to go in other directions in addition to out the magazine well.
The magazine well on the lower was bulged out. Kind of like an Elmer Fudd cartoon shotgun. The upper receiver was also bulged out from the explosion.
The bolt and carrier were both trashed – bent all to hell and completely stuck in the upper and barrel extension.
I assume the barrel extension and barrel were trashed, but as everything was fused together, there was no way to tell for sure until they rip things apart. Shoving a .308 inch diameter bullet into a .223 inch hole is asking for damage I would think.
While I was not shocked at the damage to the aluminum upper and lower, I was surprised at how much the bolt carrier and bolt were trashed. That’s hard stuff there.
Yeah, it’s hard stuff, but a 5.56 NATO load is already creeping close to the limit load of the system, with respect to chamber pressures. Eliminate the possibility for that load to be tapped off by a gas route out of the chamber and down the barrel, and bad things happened.
With the brief opportunity I had to look, that’s about all I could tell. But now I was curious. Would similar rounds allow the .223 rifle to go into battery? I decided to try under much safer conditions.
And what he did was remove the BCG from a 5.56 rifle and see if a .300 BLK would drop into the chamber. The answer was what we think of as The Universal Answer to Everything™: “It depends.” In this case, it depends on the bullet; any .308 bullet can be loaded in the Blackout, with the lighter projectiles for maximizing velocity and heavier projectiles for subsonic use with a suppressor. Result of his experiment: A small, high-velocity bullet in the .300 would chamber, at least, most of the way; a large, subsonic bullet (200+ grain) wouldn’t.
In case you’re wondering why the US .mil doesn’t use the .300 BLK, this is one answer. Captain Murphy’s law always was, “if anything can go wrong, it will,” and while the original Murphy was a flight-test engineer, he sure as dammit could have been a weapons man with an insight like that. If it is physically possible for Private Joe Snuffy (or his Marine opposite number, Lance Corporal Schmuckatelli) to assemble a firearm improperly, or load it improperly, he is absolutely going to do it. Like the poor bastard in the example above, who was fortunately not seriously injured.
People I know who do use ARs in many calibers don’t take advantage of the capability to reuse the mags with multiple calibers. It’s just asking for trouble — better to dedicate mags to special-purposes like .300 or, say, blanks. (It is very embarrassing to fire a live round with a blank-firing adapter on the rifle, and it usually totals the rifle).
Blue=Inert, standard NATO/US code color. You can get anodized mags in several different colors. As long as you pick a system and stick to it, you won’t fire the wrong thing in the wrong place.
Go ahead, whine about magazine prices. What about replacing a whole AR like the fellow whose misplaced .300 round trashed his rifle?
One last thought. We are not fond of the Forward Assist, a gadget that was added to the M16A1 very late in the adoption game, at the insistence of armchair ordnancemen who had actually used the same reason (“lack of positive bolt closure”) to reject the T48 (FN FAL) in favor of the T44 (developed Garand that became the M14). And here is one reason not to be fond of the FA.IF you are forcing the bolt carrier into battery, why are you doing that? It just might be that you have the wrong round chambered.