Like any gun and cartridge combination, the 300 AAC Blackout has its own personality. Getting to know a few of its character traits can save you a lot of time should things start to act wonky. Let’s take a look at a few of the more common potential gotchas.

Chambering the Wrong Caliber!

You hear internet stories about someone who stuffed the wrong caliber cartridge into a rifle and pulled the trigger. If you’re like me, you take these legends with a grain of salt, shake your head a bit, and move on. Another internet myth right?

Well, this one is true I tell ya! I saw it happen!

I was at my regular outdoor range a month or so back, when I heard a loud bang, closely followed by what can best be described as a “panic shriek.” The scream was intense – kind of like the one Mike Bloomberg makes when he inadvertently stumbles into a Friends of NRA banquet. Looking a few benches to my right, I saw two men, one an experienced shooter, and the other a new shooter, looking dazed and confused. Actually the newer guy was looking more scared and in shock – staring at his hands as if he was surprised they were still attached. Somehow he had stuffed a magazine full of 300 Blackout supersonic cartridges into a 5.56mm rifle, managed to chamber a cartridge, and pulled the trigger.

As you might guess, the gun exploded. Literally. The bolt was bent, upper receiver bulged out, barrel extension trashed, lower receiver trashed and the barrel was now plugged with one very elongated .308 caliber projectile. The shooter was incredibly lucky as much of the pressure escaped through the magazine well. Still, there was enough force in the conflagration to bend a lot of steel and aluminum. The shooter suffered plenty of stinging and mild burns to his hands and face.

Now that I had seen it happen, I became a believer. At least with the right bullet profile, it is possible to fit a 300 Blackout cartridge into a 5.56mm / .223 chamber – at least enough so to allow the rifle to fire. When I got home, I removed an upper from one of my 5.56 ARs and tried dropping various 300 Blackout hand-loaded cartridges into the chamber to see if any would fit. With big and fat bullet profiles, no subsonic loads would come close to fitting, but lighter weight 110 and 125 profiles did in fact come pretty close to fitting in the chamber without application of undue force.

The moral of the story is that it can happen, so be careful when shooting any rifles of similar cartridge design. In this case, it’s doubtful the experienced shooter would have made the mistake, but the new shooter had no idea. Apparently his mentor was paying attention to something else when the shooter loaded the gun.

Fortunately, there are easy ways to minimize the risk. The simplest method is to use magazines of different color for 300 Blackout. Get some tan, dark earth or grey ones. Or you could use metal mags for one caliber and plastic for another. If you already have a supply of magazines, check out MagBands – silicon bands that clearly identify the caliber contained within.

Magazines

Your combination of 300 Blackout ammo and magazines are only a potential gotcha – something you should be aware of in the event you experience malfunctions.

Standard 5.56mm AR magazines have ribs on the inside that support the neck of the .223 Remington / 5.56mm cartridge case. Since the 300 AAC Blackout case is shorter than that of the .223 Remington, the magazine ribs are well forward of the Blackout’s cartridge case mouth. As a result, the magazine ribs may touch the ogive section of the bullet itself. The section of a .30 caliber bullet ogive near the ribs might just be wider than the .223 Remington case neck. This pressure can potentially impact alignment of the bullet as it is pushed up the feed ramp.

This is not something to stress about and I have not had trouble with most of my magazines. If you experience feeding problems, take a look at a loaded magazine to see if the cartridges are being pressed out of alignment by the magazine ribs. Some bullet manufacturers, like Barnes, have developed projectiles that are shaped specifically to feed properly from standard magazines. Or just try a different make of magazine – they’re all a bit different and you might have success with another brand.

If all else fails, you can try magazines designed specifically for 30 caliber bullets. For example, D&H Industries makes magazines with ribs positioned to fit a wider variety of projectile shapes.

Gas System Length

In a standard 5.56mm AR-15, cartridges perform within a relatively narrow band of performance. As a result, gas pressure performance is somewhat predictable and rifle makers can design gas ports to operate reliably. With the 300 Blackout, and its huge range of ballistic options, finding a gas system design that works with supersonic loads and bunny fart subsonic loads can be a real challenge. If too much gas is allowed, the supersonic rounds can create excessive wear and tear on the rifle. Too little and subsonic rounds may not generate enough gas to reliably cycle the action. With 300 AAC Blackout gun design, there’s a constant tradeoff between reliability and gas pressure. Start doing things like changing the barrel length and things get even crazier.

There are four gas system lengths in the AR family: pistol, carbine, mid and rifle. In that order, those descriptions simply indicate how far from the chamber the gas port is. So in a pistol length system, the gas port is maybe 4 inches down the barrel, while in a rifle length system, it’s about 12 inches away from the chamber.

Here’s the insanely practical definition of the issues related to gas system length and the 300 AAC Blackout. The AR design relies on gas bleeding off through a port in the barrel to cycle the action. While the bullet is still in the barrel, there’s lots and lots of gas pressure available for cycling purposes. As soon as the bullet exits, pressure drops like Piers Morgan’s ratings. So far so good?

With 300 Blackout and its subsonic capability, people like to do cool things like use shorter barrels, say significantly less than 16 inches. With a standard carbine length gas system and a short barrel, gas is siphoned off through a port fairly close to where the bullet exits the barrel, so there is very little time of “high-pressure gassing” to cycle the action. The bullet is gone just as things start to operate and pressure drops too soon. Here’s the value of a pistol length gas system. With the port closer to the chamber, there’s a longer period of awesome pressure time between the time the gas reaches the port and the bullet leaves the barrel. Lotsa gas and plenty of time means reliable operation of the action.

So here’s a rough rule of thumb for 300 Blackout guns. If your gun has a 16-inch barrel, a carbine length gas system should work just fine with supersonic and subsonic loads – especially if you use a silencer. It’s not a hard and fast rule. Some rifle makers, like Noveske, use pistol length gas systems in their 16-inch barrels, which other makes use carbine length. If you are going to buy or build a short length barrel gun, less than 16 inches, do yourself a favor and choose a pistol length gas system.

Hopefully these things I labeled “gotchas” don’t cause undue stress about considering the 300 Blackout. Just knowing about them might save you time and aggravation should you experience reliability or function issues.