Personality Quirks of the 300 AAC Blackout

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!

Here's a .223 Remington cartridge dropped into a 5.56mm chamber. No problem!

Here’s a .223 Remington cartridge dropped into a 5.56mm chamber. No problem!

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.

Here's a 300 Blackout cartridge loaded with a ballistic tip bullet, dropped, not forced, into the same 5.56mm chamber.

Here’s a 300 Blackout cartridge loaded with a ballistic tip bullet, dropped, not forced, into the same 5.56mm chamber.

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

Notice how the magazine ribs support the cartridge case necks with the .223 Remington cartridges on the bottom. The ribs touch different parts of these various 300 Blackout projectiles directly.

Notice how the magazine ribs support the cartridge case necks with the .223 Remington cartridges on the bottom. The ribs touch different parts of these various 300 Blackout projectiles directly.

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.

Shown here are three different brands of magazines all loaded with the same "fat ogive" 300 Blackout rounds. Notice how the will each feed a little differently?

Shown here are three different brands of magazines all loaded with the same “fat ogive” 300 Blackout rounds. Notice how the will each feed a little differently?

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.

This Daniel Defense DDM4v5 300 Blackout has a 16-inch barrel - and a carbine length gas system.

This Daniel Defense DDM4v5 300 Blackout has a 16-inch barrel – and a carbine length gas system.

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.

This Daniel Defense ISR has a short barrel, integrated silencer and a pistol length gas system.

This Daniel Defense ISR has a short barrel, integrated silencer and a pistol length gas system.

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.

{ 13 comments… add one }
  • Matt August 4, 2017, 8:17 am

    I’ve recently built a 300blk rifle, 16 inch barrel with carbine length gas system, that is failing to cycle/eject properly. Thr spent casing is getting mashed up into the upper. Anyone else have this problem and find a fix?

    • JD August 21, 2017, 11:32 pm

      It’s the gas system. 300 black works optimally with a pistol length gas system even with a 16″ barrel from what I’ve heard

    • Kevin November 2, 2017, 12:32 am

      1.Completely clean your 16″ barreled carbine gas system rifle.
      2.Check your gas block./tube alignment for a restriction/perfect seal.
      3.You should be running a carbine buffer(2.9/3.0 oz.)
      4.Ensure your buffer spring is not binding(a little grease will quite it down too)
      5.Your gas block port diameter should be around .156″.
      6.Your barrel port diameter should start at .093″.
      7.If your experiencing this failure to eject/feed/chamber while shooting exclusively subs(208-220gr)/custom light loads – “no chit” .
      8.Swelling your barrel port ONLY(not your gas block) a few thousandths at a time(number size drill bits .093 – .125) until you can successfully cycle all types of ammo.
      9.The trick is to NOT over gas your supersonics while trying to get your subsonics to cycle.
      10.I also assume your not having any BCG issues(broken/misaligned rings),running the rifle dry,steel case ammo(please),
      & head space issues.
      11.A 16″ barreled 300 BLK is on the ragged edge of a carbine length gas system(and 300 BLK max co-efficiency/ max range) but most store bought rifles are built with them.Anything shorter than 16″ is definitely pistol gas neighborhood.
      12. I’ll bet your ready to clean your rifle now.

  • John October 8, 2016, 8:00 pm

    Great write up! I’m fairly new to learning about the .300 blackout cartridge. Excellent info for sure. I’m in the process of finishing a build and since I’ve already got two in 5.56 (one a pistol and the other 16″ barreled rifle) I decided to finish this upper in .300 I now have a better understanding of why I’m seeing that most of the barrels in the 16″ length I’ve seen are in a pistol length gas system. All that said, it appears that from the comments above, I should be ok with a 16″ barrel running sub or super without a can right? I haven’t started the long process for owning a suppressor/silencer yet. But I will!!

  • Jake December 2, 2015, 11:41 am

    I just bought a 16″ 300 blk upper for my brother. He’s going to drop it in his existing ar lower. It’s pretty standard and we aren’t planning on attaching a can. My concern is that it is using a pistol length gas. From what I’ve read, it’s optimal for suppressed 300 blk to have that length gas system, but will it be ok to run it unsupressed, with supersonic rounds? If so, what grains and loads are recommended?

    • Chuck August 25, 2016, 11:45 pm

      Jake, I don’t see a date on the comments, so you’ve probably figured it out already.

      The suppressor will increase the back-pressure, so a longer gas system might work on a suppressed barrel but short-stroke when it is taken off.

      This is especially true with subsonic loads which are lower pressure to begin with and the pressure drops rapidly the further you get from the chamber.

      • Ken December 11, 2016, 6:03 pm

        Chuck, as stated…you probably have this figured out already. On a side note, my Uncle and I just build/assembled a pair of “twins” in 300 BO.
        Once assembled (16″ barrel/carbine length gas system ) we test fired my Uncle’s, and the round wouldn’t properly eject. We swapped buffers and springs, BCG, and even hand loaded a variety of rounds…all with no success in ejection. Once we took the upper apart, found that the gas tube wasn’t perfectly lined up with the gas block. Furthermore, the gas tube diameter was significantly larger than the gas block port. Taking it a step further, the barrel gas port wasn’t near the diameter of either of the aforementioned holes…leading to a significantly undergassed AR.
        We proceeded to drill out our barrels, and the gas system @ the gas block…and ultimately got our AR’s to reliably eject and feed a variety of loads….without incident.
        We don’t run a can, or subs.
        I’m posting this up for those who might run into this problem with their own builds….After this post.
        Good luck and have fun.

  • Mike August 19, 2014, 5:00 pm

    Great article! Had a friend encounter a baffle strike when shooting factory subsonic. Wasn’t an enjoyable experience especially since it was a brand new can!

    • Tom McHale August 19, 2014, 9:21 pm

      I blew a can off an M1A once… Still not sure exactly how that happened, and amazingly, neither the can or rifle was damaged. #StrangeThingsHappen

  • JimBob August 18, 2014, 10:08 am

    Tom,
    Thank you very much for a very well written and even more informative article. I’ve built quite a few 300’s and have had great success with this round. I did have one customer that experienced a squib where the round lodged at the end of the muzzle and he continued to shoot until the barrel was full of lead (pretty scary). When he brought the rifle for me to look at I was amazed he still had fingers and eyes. I sent the deformed, but intact, barrel to FN to show them what a great and safe product they produce in the CHF triple chrome lined barrel. And yes, marking the magazines is a great little bit of insurance.
    Thanks again,
    JimBob

    • Tom McHale August 19, 2014, 9:20 pm

      Whoa! Amazing it didn’t blow up entirely. Must have been subsonic?

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