Avoiding the .300BLK AR-15 Rifle Ka-Boom

Note: This article was originally posted on NRA Blog: https://www.nrablog.com/articles/2016/9/avoiding-the-300blk-ar15-kaboom/

.300BLK AR-15 Ka-boom
.300BLK AR-15 Ka-boom

USA – -(Ammoland.com)- Spend any amount of time scouring gun boards and blogs and you’ll undoubtedly come across pictures, accounts and horror stories of the dreaded AR-15 “ka-boom!” caused by chambering the wrong ammunition in a rifle, resulting in shattered rifle receivers and shaken-up shooters.

Most frequently as of late, those ruined rifles are due to accidentally loading a magazine full of .300 Blackout into a .223/5.56-chambered AR-15, an easy-to-make but dangerous mistake that can have dangerous consequences.

Take a look:

https://da8g8yhtnvjyi.cloudfront.net/media/1538438/kaboom-2-oncefired-pb.jpg (Photo courtesy/300BLKtalk.com)

This too…

https://da8g8yhtnvjyi.cloudfront.net/media/1538439/kaboom-bearingarms.jpg (Photo courtesy/BearingArms.com)


It doesn’t need to be that way, however. Using the correct ammo for the firearm you’re shooting is an absolute must. By taking extra steps and attention to detail when loading your magazines and your firearm, you’ll be able to avoid damaging or destroying your gun due to chambering the wrong ammo.

A little background on the .300 Blackout round: With its earliest roots in gun designer J.D. Jones’ .300 Whisper wildcat cartridge, the 300 .AAC Blackout was created by Advanced Armament Corporation in cooperation with Remington Defense to provide a specialized .30-caliber round to meet the needs of special operators who wanted the performance of the .30-caliber projectile while using the equipment they already had, namely the M16 rifles and M4 carbine.

https://da8g8yhtnvjyi.cloudfront.net/media/1538432/40cg.jpg (Photo courtesy/40CG.com)

The .300 Blackout allowed users to simply retrofit a .30-caliber barrel to their rifle without making any other changes, as the .300 Blackout uses a .223 case cut down to accept the .30-caliber bullet, meaning the bolt and magazine for a .223 or 5.56 firearm would work for a .300 BLK round.

Therein lies the potential for misuse.

Always – ALWAYS! – visually inspect your rounds

If you’re anything like me, loading your magazines at the range can be a relaxing exercise in repetition, especially knowing you’re about to partake in some usually much-needed range time (we could all use more practice!). However, without ammo discipline, it’s way too easy to accidentally load a 300BLK round into an AR-15 .223 magazine – they’re DESIGNED to fit, after all.

If you even as much as own any .300BLK ammo and plan to load and shoot .223 or 5.56, it’s a prudent practice to visually observe the rounds you load into your magazines. Even though the parent case is the same and the rounds are the same length and diameter, you should be able to instantly distinguish the two rounds due to the difference in the size of the projectile.

Carefully observing each round you insert into your magazines will help you avoid mistakenly loading the wrong round into the firearm, in turn preventing a catastrophic failure and likely destruction of your rifle. This is particularly important, as you cannot rely on your firearm to tell you you’ve loaded the wrong ammo — the .300BLK cartridge will chamber in a 5.56 rifle, but in no way, shape or form should you ever try to fire it. The .22-caliber bore simply isn’t big enough to handle the .30-caliber projectile of the .300BLK round, and all that pressure from the round needs to go somewhere once the primer is struck.

https://da8g8yhtnvjyi.cloudfront.net/media/1538434/chamber-test-oa2-org.jpg” width=”800″ height=”600″ /> As you can see, a .300BLK round will chamber in a .223 gun but under no circumstances will it fire correctly. (Photo courtesy/OA2.org)

Labeling, Coloring and Separation

While you should always inspect the rounds you’re loading into your magazines, but another way to practice good ammunition discipline is to keep different calibers in separate containers that are clearly marked to describe what’s inside.

It may be bit cumbersome to drag multiple ammo cans or boxes to the range, but doing so could serve you well when it comes to staying safe on the firing line. Consider using different colored ammo boxes for different calibers, using paint, markers or stickers to label each with the caliber they contain, and maintain physical separation of the containers as much as possible.

https://da8g8yhtnvjyi.cloudfront.net/media/1538437/j-and-j-products.jpg (Photo courtesy/J & J Products)

Another good tactic is using different magazines dedicated to a specific caliber. For example, I used solely Magpul magazines for my 5.56 ammo, while opting for traditional metal magazines for my .300BLK rounds. The difference in appearance and texture can serve as a tactile reminder of what round you’re about to chamber.

Other shooters have used different colored magazines to differentiate between calibers, like loading standard black magazines with .223 and reserving flat dark earth mags for their .300BLK ammo. More subtly, some shooters adorn their magazines with colored tape to make the designation clear.

Some companies create labels or decals you can apply to your magazines that specify what caliber it’s used for, and even Ruger took the smart step of labeling its .300BLK magazines bundled with their .300 Blackout barrel kit for their SR-556 Takedown rifle.

https://da8g8yhtnvjyi.cloudfront.net/media/1538436/gunsamerica-ruger.jpg (Photo courtesy/GunsAmerica)

Whatever method you choose is great as long as it helps you and fellow shooters recognize the differences and practice good ammo discipline.

One at a time

Perhaps the most drastic but effective way of avoiding catastrophe is to simply shoot one caliber per range trip. Sure, it may be overboard to limit yourself, especially if you can’t get to the range that often and want to take advantage of all your guns when you do, but only bringing one of the two calibers to the range – with the correctly chambered rifle, of course – seems a great way to prevent accidents from switching back and forth between calibers.

If you must take both calibers shooting and want a little extra peace of mind in addition to the above steps, consider securing your alternate calibers and firearms in a locker or storage area away from the firing line until you’re ready to use them. Doing so creates further delineation between 5.56 and .300BLK.

Observe and intervene

Even if you have all your ducks in a row and practice fail-proof ammunition discipline, that doesn’t mean everyone around you is as educated or dedicated. If you head to the range with family or friends and plan to bring both .223 and .300BLK, make sure they know the difference between to two, the dangers of accidentally mixing them up in the wrong rifle, and understand your system of differentiating between the two calibers.

Short of loading everyone’s magazines for them, it’s a good idea to observe others as they interact with the firearms, particularly inexperienced and younger shooters. Some might be annoyed or nervous by your attention, but it’s much better than the alternative of someone making a mistake that could lead to a broken rifle or worse.

https://da8g8yhtnvjyi.cloudfront.net/media/1538441/300-blackout-vs-556-featured.jpg (Photo courtesy/StuntGunner)

The .300 Blackout is an innovative cartridge that bridges the gap between intermediate calibers and full-power .30-caliber rounds, providing flexibility and performance in a familiar and lightweight package.

Safety should always be your foremost concern on the range, and that definitely applies when using .300BLK and .223 together. A little discipline can go a long way in preventing a dreaded AR “ka-boom.”

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I’m in a blended family, 5.56/300blk. My solution is…the 300 rifle looks different, the magazines have yellow duct tape around the bottom for visual and tactile identification. I’m going to invest in a bunch of silicon bands…JUST for the 300. I have a friend that just doesn’t quite seem to get the difference between the two. Needless to say, I’m very very very careful around him which is a touch strange because he is very detail oriented. Just doesn’t quite get the difference no matter how many show and tells I do.


Though not nearly as dangerous I’ve seen people at ranges shooting 9mm ammunition out of .40 S&W semi-auto pistols. They wonder why they can’t reload their split 9mm brass?


Awakened by midnight intruders?
Combat jitters, frayed nerves?

My SIG 556R (7.62×39) uses AK mags.
My SIG M400 uses AR mags.
My Ruger Mini14 Tactical uses Ruger mags.

No problems.


I won’t go so far as to call people who have this happen “idiots” but I can say with certainty I would never shoot with that person. All of the tips listed in this article require the one thing that none of these people used – attention to detail. You can talk about the psychology of repetition all you want, it does not change the facts. When you have a hobby that can potentially kill you when things go south, you MUST PAY ATTENTION 100% of the time. Best case scenario you blow up the gun, I don’t need to… Read more »


Totally agree with attention to detail. But I also would like to add in respect. Once you lose respect for those items you’re dealing with and The lethality that they have then you will make the mistake of loading the wrong ammunition in the wrong gun or riding the motorcycle faster than the conditions allow and the list will go on and on. Attention to detail and respect for those items you’re using will prevent stupidity.

Zulkiflee Dawod

I like HK416, 417 and SCAR with taliscoop, M79 launcher how would be the price.


Given that I have grown children and grandchildren that will likely do over 90% of their shooting when they come to visit me, I need a means of simple recognition discrimination. I bought red mags for my .300 BO and had the barrel cerakoted red between the suppressor threads and gas block surface. Will have the .458 SOCOM mags and barrel similarly coated gold. Any 5.56 will remain black. Additional calibres will get their own color. Will get ammo boxes to match and segregate ammo accordingly. Hunting with any of these calibres is usually a night time affair, so bright… Read more »


Since I’ve never tried it (or had it occur in my experience), is use of the bolt -assist required to chamber the 300BLK into a 5.56?, and is it consequently stuffing the projectile into the propellant chamber to close the bolt?
Can such a kaboom happen as an OBD while dropping the bolt using the charging handle?
Do you have to manually -cram- the round in and then pull the trigger to cause the catastrophe?

Charlie Valenzuela

Yep. You’ll have to cram the round into the chamber. It isn’t going to go in by itself or from the force of the bolt slamming home after firing a 5.56 round first. Despite the photo showing that the cartridge will [sort of] chamber in a 5.56 upper, the fact is that the 300BLK is about 3/16-inch too long for the chamber (unless you are firing a crimped blank), so there will definitely need to be some tapping done on that forward assist. Some SERIOUS tapping. Nevertheless, I am certainly aware that accidents do happen and in an attempt to… Read more »

Mike Murray

I’ve seen many, many times where a 9mm had made it into a .40, or a .40 had been loaded into a .45. While it’s no big deal with a semi-auto (other than causing a hard malfunction requiring a cleaning rod), it is indicative of the careless and casual mindset some shooters have. Usually, it’s caused by having multiple magazines and multiple calibers open on the same bench, at the same time.

Worst Case

Thx. Good to know, be reminded. Shivers thinking of that happening near my face. More shivers thinking of that happening with a poly lower.


Bought red magazines for my .300 BO, then had the barrel cerakoted red between the suppressor thread and the gas block surface to match. Will do my .458 SOCOM barrel and mags a different, bright standout color. All 5.56 barrels and mags to remain their original colors. Bright colors are not likely to be noticed by feral hogs in the wee hours at the rangees they are likely to be encountered, and most prey animals are colorblind to boot. These precautions are for my family members to be more easily trained as they do not live here and only fire… Read more »


Buy some cheap “nail polish” at the Dollar Store/WalMart, etc. Use dots of the polish to “color code” the UPPER and the magazines. In my case, red is 300 Blk, blue is 6.5 LBC/Grendel, white is 7.62×39. ( Actually, the Grendel and 7.62 magazines could carry two “dots” since the magazines are interchangeable, and there is no danger of the incorrect round being fired in the wrong upper.)

David Christensen

All my black guns (AR-15) shoot 5.56. It the gun shoots any other caliber, like .300 AAC Blackout,or 5.6 Grendel, I Cerakote the weapon (and the mags) Flat Dark Earth. (My 6.5 is done in 3 tone Kryptek) The ejection door also says .300 or 6.5. Even though the mags are a different color, they are still stenciled with the proper caliber.. My .308 is a black gun, but it is an AR-10. Even if I remember to look at the rounds BEFORE insertion, I want to make sure my family members are also safe, and I explain the process… Read more »


Their’s a concept called Poka-yoke (ポカヨケ?) [poka yoke] which is a Japanese term that means “mistake-proofing” or “inadvertent error prevention”. It’s difficult for me to call someone a, “Stupid Moron” when it’s just human error. Shooting is a highly repetitive task and therefor particularly prone to lapses of attention. The human mind doesn’t deal well with highly focused repetition over time, and the design of the .300 just invited mistakes. I have the Wyndham AR chambered in 7.62×39 which operates flawlessly, but would never allow such confusion because the possibility is engineered out of the system. I know I’ve given… Read more »

The Old Coach

Having served a 15 year sentence in Michigan, engineering automation for the Big 3, it’s my considered opinion that the automakers (and OSHA) would “poka-yoke” this situation by forbidding any .300AAC rifles or ammo on the same premisis as the standard 5.56×45. If ya gotta have a .30 caliber upper, do like A.J.

Mike McAllister

Have to agree KCshooter. If you don’t know what your gun is designed to shoot you don’t need to be shooting it!!!!


Every .300blk kB! in a 5.56 gun that I’ve seen so far has been entirely the result of the stupidity of a moron. Period.