A Cautionary Tale: Loading .300blk Into 5.56 AR

Baraka of MASF shared this with me. A MASF member, Carlos Santillan shared an experience of a catastrophic failure. Thankfully no one was hurt. Carlos wants others to learn from his and other’s mistakes.

– Background:
I was at a friend’s home to try out his .30 cal suppressor on my SBR. I have the same make and model suppressor waiting on the approval of my Form 4 (about 2-months to go). My friend had just obtained a relatively new 10.5″ upper that came with an Aimpoint T2 for a decent price. When I found out that his whole family was going to be home and he was firing up the BBQ grill, I offered to bring my (registered) select fire AR and .22LR sub-caliber device for them to shoot. After I let the whole family, plus one of the sons’ friends, run 325-rounds of .22 LR ammo, we recovered back to the patio where I got to test out his .30-cal can on my 10.5″ 5.56 SBR.

– Situation:
After I was done, my friends’ youngest wanted to do some shooting with him. So I stood back while his son shot as suppressed M&P-15-22 at some water bottles filled with colored water. Once the youngest got tired (or frustrated?), my buddy decided to give his new upper a try while suppressed. As I was removing his (still hot) .30-cal suppressor from my SBR, his oldest son, a very hard-headed, 18-year old, recent HS graduate, know-it-all with an attitude, came out of the house and “declared” that he wanted to shoot the new gun (MISTAKE #2) (I will mention MISTAKE #1 later). He put on a set of ear muffs, picked up the rifle, grabbed a loaded magazine off the table and abruptly asked his Dad if he had the right “bullets” (MISTAKE #3). My friend quickly looked at them and nodded yes (MISTAKE #4). I observed the rounds loaded in the mag had red ballistic tips and was wondering why he was shooting those instead of FMJs (MISTAKE #5 – mea culpa).

My buddy and his son went to the established firing point and his son loaded the rifle without waiting for any command (MISTAKE #6). He took aim at the target and BOOM!! A huge ball of smoke engulfed the rifle and I observed pieces flying away from the gun. When I took a closer look at what hit the patio floor, I recognized parts and pieces of the gun and its magazine. I informed my friend that the gun had just blown up. He gave me that “I don’t believe you” look and they dropped the magazine (a windowed PMag) and tried to unlock the frozen action.

As I was watching them struggle with the rifle, I glanced down and noticed that one of the items that hit the ground was a piece of brass that looked kinda odd. I reached down and picked it up and realized what MISTAKE #1 was – the magazine his son picked up was loaded with .300 Blackout ammo!! I had not realized that my friend had brought out his .300 Blackout AR and placed it on the same table as the 5.56 ARs. In his son’s rush to do things his way now, and his Dad’s decision to let him get away with it just to avoid a confrontation, nobody had the opportunity to do establish and follow the proper procedures thus resulting in a catastrophic failure of a new upper receiver and some (fortunately) minor injury.

MISTAKE #1 – Segregate and clearly mark your different calibers before you get on the firing line. This is especially important between “related” calibers like the .300 Blackout and the .223 REM/5.56x45mm. As in this case, the .300 Blackout will chamber in a 5.56 – once. The same should go with the corresponding rifles. Consider dedicating your AR mags to a particular caliber and marking them accordingly. Mark all the magazines you plan on using for anything but a 5.56 AR. I personally like to use either red or yellow electric tape so that I can easily tell at a glance that the magazine is not meant for my 5.56 AR; I use a marker on both sides to write the actual caliber. The same goes with other calibers (.308/7.62 vs 6.5 Creedmoor). Wal Mart sells those plastic “.30 Cal” ammo cans for about $5.00. Dedicate one for each specific caliber and also make sure you mark them accordingly.

MISTAKE #2 – Just because you have given up on disciplining your “adult” child, it doesn’t mean you should not keep them in line, especially on the firing line. Don’t be afraid to tell anyone to hold off and step back until you can properly organize yourself and the range. The firing line is not the place to be a “nice guy”. I would rather share the line with a safe asshole, than someone unsafe.

MISTAKE #3 – If I were running the line I would have had the kid put down the rifle and magazine and have him step back until I was good and ready to deal with him. I have run into this very situation when running a range with to two dozen shooters, or more, vying for their turn to use a particular barricade or shoot at a particular target. I’ve actually stopped the whole line and explained to everyone that we are all going to take a timeout and had them all unload and ground their guns while we did. I took the opportunity to cite unsafe practices and attitudes. There is no reason to be rushing anything at the firing line.

MISTAKE #4 – When it comes to verifying the correct caliber being loaded into the correct firearm, take the time to really check. Even if you segregate and mark the magazines and ammunition as suggested above. Someone could easily make a mistake and mix up your organization.

MISTAKE #5 – I should have questioned if he meant to shoot what is obviously ballistic tips (which he uses to deal with raccoons, foxes and coyotes that go after his chickens) versus FMJs that he uses for training. This might have given him pause to more closely inspect them in detail and could have prevented the mishap.

MISTAKE #6 – Unless I give a shooter to load and engage at their leisure, I have been known to stop someone, have them unload and make clear, and have them wait until I am clear on what their intentions are. When I was letting the family shoot F/A, I maintained positive control of each and every magazine that was inserted into that gun. It didn’t matter that they could never have loaded any other caliber since the only source was a can of Federal .22 LR that I supplied. Even with the oldest son, I took physical control of the magazine and inspected it before I let him load it into my gun.

– ADDITIONAL LESSON: I have never had the urge to purchase a rifle that was chambered for the .300 Blackout. Once in a while, I’ll come across one that is selling for a song and the “Good Idea Fairy” will make an appearance. But then I realize that I really have no need for one and would rather not another caliber to my collection. After this experience, the chances of me doing so is even less.

I hope this helps someone down the line. Stay safe!!



Born and raised in the Philippines under Martial Law, Carlos Santillan first immigrated to the West Coast of Canada where he purchased his very first firearm, a Charter Arms AR-7 .22LR pistol. Carlos eventually moved to Baltimore where he applied for and became a Resident Alien. Three days after receiving his Green Card, Carlos enlisted into the US Army as an Infantryman with the Airborne School option. He first served as an Indirect-Fire Infantryman in an Armor Battalion; spending the last 26-months of his 4-year enlistment as an Arctic Paratrooper.

Upon receiving his Honorable Discharge, Carlos transferred to the MD-ARNG where he eventually completed the State Officer Candidate School (OCS) program and was commissioned as an Aviation Officer. After rotary wing flight training, he was assigned as the Aeroscout Platoon Leader and Executive Officer in an Attack Helicopter Battalion, along with the additional duty as the Battalion Marksmanship Officer.

After 8-years of service in the Army National Guard, Carlos transferred into the Inactive National Guard and subsequently the Inactive Ready Reserve. Eventually joining the US Army Reserve as a Civil Affairs Officer, served for 4-years in various positions, eventually leaving the military after almost 30-years of combined service in 2013.

In addition to his military service, Carlos spent 9-years as a Volunteer Deputy and more than 15-years volunteering as an NRA Certified Basic Pistol Instructor at his gun club. He also founded and, up until his decline in health, ran that club’s Tactical Shooters Group – a group of more than 60 shooters who prefer to conduct tactically-oriented drills. Carlos holds a Bachelor’s Degree in Information Systems Management and an MBA in Information Security.

Nicholas C

Co-Founder of KRISSTALK forums, an owner’s support group and all things KRISS Vector related. Nick found his passion through competitive shooting while living in NY. He participates in USPSA and 3Gun. He loves all things that shoots and flashlights. Really really bright flashlights.

Any questions please email him at nicholas.c@staff.thefirearmblog.com


  • Duke of URL

    Should’ve included a photo of the blown-up gun…

  • Lee Attiny

    My take away from this is don’t have kids and If you do at least raise them not to be dipshits.

    • FUBO

      Yeah,let theMuslim hoards out produce us.

      • Sgt. Stedenko

        Keep drinking the MSM Kool Aid.

        • FUBO

          No Kool Aid here douchebag.

  • kamen_volk

    I thankfully only have two calibers for all my guns, 9mm and 5.56/.223, so I don’t have this problem, but thanks for posting about your mistakes. It helps everyone grow as we learn from each other’s mistakes.

    • KestrelBike

      No shotgun?

      • Cymond

        No rimfire?

        • Sunshine_Shooter

          No belted magnum?

  • Sianmink

    Personally, I keep all my .223 exclusively in Pmags and my .300 BLK exclusively in smoke Lancers. That makes it really really easy to tell the difference.

    • 1911a145acp

      How do you tell the difference between nearly identical polymer mags in low light/ darkness? There MUST be a visual AND tactile indicator. All my 300 BLK mags are Black windowed P-mags with Ranger mag pulls on the bottom, 5.56 mags are non windowed and flat bottom. A fellow shooter only loads 300 BLK in 20 round mags, and 5.56mm in 30 round mags. I have been shooting 300 Whisper and 300 BLK for 7 years now, it is VERY easy to screw up when 5.56mm & 300 BLK ammo, mags, and guns are in close proximity. I STRONGLY encourage shooters with both calibers in ARs to create visual and tactile indicators on the mags and make very deliberate efforts to keep the two calibers seperated. If it doesn’t load right or feel right DO NOT force it- double check before you fire. One shooter repeatedly banged the fwd assist until he pushed a 300 BLK far enough into the case until it fired in his 5.56mm chamber. It did, and destroyed his rifle.

      • Sianmink

        I’m pretty sure I can tell the difference between the two by touch, since they have distinctive texture zones. Lancers have nubblies on the sides and smooth spines while the Pmags do not. If I determine it to be a problem, I’ll add some tape.

        • Mikial

          “I’m pretty sure I can tell the difference”

          Ahh, famous last words.

  • Bill

    Pardon my ignorance, but:
    1) How can a .30 cal. bullet seat far enough in the chamber of a 5.56 to actually be “fired?”
    2) What is an “Arctic Paratrooper?”

    • Giolli Joker

      1) if there is enough space behind (likely with a lightweight. 300 blk load) once the bolt slams the bullet against the barrel throat, it may be pushed further deep in the case.

      • Bill

        Thanks, I never would have considered that. I don’t reload or mess with cartridges at all, other than getting them out of the (right) box and shooting them.

    • Cymond

      Yeah, 7.62 is larger than 5.56, but the case is shorter, the 7.62 bullet sits further back, and the shape of the 7.62 bullet is comparable to the neck & shoulder of the 5.56.

      • Tom Currie

        It STILL won’t chamber — at least not without driving the bullet back into the case, as Giolli suggested.

        • Twilight sparkle

          This isn’t the first time a 300 blackout round was set back far enough to be fired though. Another possibility is that these could have been reloads with bullets that were light enough to not interfere with the 5.56 chamber

        • FarmerB

          This piece just goes to show you that at least one place in the universe and at one time, you’re wrong.

          • flyfishr

            How is he wrong? There are zero details to disprove that it was exactly as he described. There are zero details at all.

          • FarmerB

            I think you’re too thin skinned – you are the one typing in capital letters after all. You world will be a safer place if you worked with the premise “what can happen” rather than “what should happen”.

          • flyfishr

            Starting a sentence with a capital letter offends you?

            You must be the reason my tooth brush package says not for rectal use, because it can happen.

          • Tom Currie

            No, but your nonsense insulting reply just shows that you need to learn to read. Go find someone who CAN read and have them read my post to you — be sure to ask them to explain those complicated big words like “at least not without”

  • Anon

    This is a lesson that needs to be taught? Really?

  • Each birthday I know less

    The young are born knowing so much.

  • DIR911911 .

    no pictures?

  • roguetechie

    This is a very valuable lesson, and I’d like to point out something.

    Outside of aviation hobbyists there’s basically no other group who share so willingly and frequently tales of their own mistakes and lessons learned!

    This is something to be very proud of, and a tradition to continue building on.

    It puts the lie to our portrayal as dangerous, uncaring, negligent, and selfish in the media whether they acknowledge it or not…

    Selfish people are prideful people and prideful people don’t share their stories of what went wrong. That would involve embarrassment and require acknowledgement of a deficiency in what they were doing or how…

    • Matt Wilder

      I chuckled when you mentioned aviation hobbyists, such as myself. Both the R/C guys, and the full scale guys (I’m involved with both as well as firearms and music as my central hobbies) sometimes are so honest about mistakes to a point, that you can easily get into a pissing match about whose safety lesson story tops whose; all in fun of course. Let’s face it; anything with a spinning prop, a jet intake, or a rotor spinning at several hundred to several thousand RPM is just as effective as any bullet when it comes to making a good day bad. And just as firearms, if you don’t take safety and situational awareness to the extreme, there can be deadly consequences for not only you, but other’s around you.

      However, musicians, and us singers and us guitarists especially, will NEVER own up to mistakes, and have every other excuse in the world for doing poorly or even being unsafe. It’s called RDES; “Racecar Driver Excuse Syndrome”.

      • roguetechie


        I sorta keep track of the RC world (mostly because it’s an awesome parts source for things like 3d printers and etc that are small or cheap) and flew rc planes with my dad way back when.

        I like hobbies that encourage self reflection and brutal honesty about your own mistakes and sometimes the mistakes of people around you.

        Between teaching you to look at yourself before accusing someone else, and teaching you to actually do the hard thing and not let a friend family member or strangers negligence slide (but doing it POLITELY and hopefully in a way that’s more about extraction of a useful lesson for all, not just recriminations)

        These are useful skills to a truly functional and happy life.

  • roguetechie

    Ugh sorry to double post but I wanted to add more and forgot.

    Shooting sports and everything up to and including building your own guns and reloading ammunition is a world that quickly teaches people to step back and look for what they did wrong in the aftermath of any incident.

    This way of looking at the world strongly inculcates a strong sense of accountability, personal responsibility, and self awareness on people who do it that just isn’t present in the world at large.

    As a group the majority of us refuse to just marginalize times when things go wrong but turn out OK… To us, every incident is a chance to review what we do and why. If necessary we modify it to suit what we learned and attempt to be and do better regardless if the incident happened to us or someone else.

    Good job guys, the way people respond to articles like this make me proud to be a gun guy.

  • flyfishr

    The lack of details is kinda frustrating. I have only heard of this happening with the lighter 105-125gr rounds, yet the pictures are larger 205+. Leaving out the pertinent info tells us nothing.

    • Giolli Joker

      Well, these two sentences : ” I observed the rounds loaded in the mag had red ballistic tips” – “I should have questioned if he meant to shoot what is obviously
      ballistic tips (which he uses to deal with raccoons, foxes and coyotes
      that go after his chickens)” make us lean towards 110-125 grs loads.
      However the story has plenty of details on what happened, the mistakes done and how to avoid them.
      The weight of the bullets is not central to the issue.

      • Twilight sparkle

        The weight is important though because 300 blackout will usually not chamber in a 5.56 chamber unless it’s with a lighter shorter bullet. It would be more of a reason to use heavier 300 blackout cartridges if you have a gun chambered for it

        • Giolli Joker

          What I meant is that the story warns you to be careful, the same rules of caution should apply regardless the bullet weight.

          • flyfishr

            He spends to much time blaming others and patting himself on the back for how safe he is. Thankfully others that have had this happen know to provide real information so the problem is understood and not just written an opinion piece like this guy.

            So you know, the 208gr amax has a red ballistic tip which you would use with said suppressor, so he really provided nothing in the way of details.

      • 1911a145acp

        Nosler ballistic tips are color coded. I’m not sure why he said red tips ( he may be mistaken on the brand ) because 30 cal Nosler ballistic tips are ALL green. It MAY have been Hornady projectiles, most all thier bullets have red tips. He may have been trying to say that the bullet tip COLOR was wrong in any case for 5.56mm ammo that he had on hand and he should have caught it. It cannot be over emphasized, if you have mult-caliber AR platforms with mags, ammo and different guns that have EVER been in close proximity, you MUST double check before you load and fire, every time- it just takes a second. Some time ago, I was in a Carbine class with a renowed trainer who had just recieved a prototype 5.56mm carbine that bear’s his name. He generously and graciously allowed us students to shoot it while he left the range for lunch. So the ONE jack ass running an AR in 5.54×39 jams a round in the chamber and we worked for 45 minutes to get it out before the Instructor came back. Don’t be THAT guy, and don’t blow yourself up……..

        • uisconfruzed

          My 300’s fed with Hornady 125g & 208g red tips.

    • Eric

      Don’t cry and be hurtful of the man.

      • flyfishr

        Cry? Not the smartest cookie in the shed are you.

  • Cory C

    Carlos, thanks for sharing with us. Also, thanks for moving to America and then promptly putting yourself in harm’s way to defend her. My hat is off to you, sir.

  • MrFN

    This made me both cringe and chuckle. I’m 18, and recently graduated, but when I was done building my AR I knew that this was a brave new world and I knew next to nothing. Over the past couple of months, I’m pretty sure I’ve become a safety nazi. People seem to forget that they’re basically holding a controlled explosion, glad no one got hurt, hope that doofus learned a lesson in patience and situational awareness.

    • Sunshine_Shooter

      I’m with you, fairly new to this and a total safety nazi. I just hope that you and I don’t become complacent.

  • guest

    I think he forgot something in his “lessons learned” list:

    – Mistake #1: never purchase a redundant calibre gun or upper

    • #7 Never listen to guys who put rules “never” on items of judgement.

      300 BO is useful. And redundant calibers can be fun. For reloaders like me, the oddball calibers are a way to get otherwise expensive guns for fractions of their value in common calibers.

  • Rock or Something

    On the range, everyone’s a safety.

  • Tonewall Jaxon

    my 3oo has 3 written all over the mags and the other guns NEVER come out when i’m shooting it…….which is 95% of the time now….

  • MattCFII

    One other thing I do with 300BLK ARs is only take 300BLK or 5.56 ammo and rifles on that range trip, I never take both calibers at the same time. I only use 300BLK in my sand PMAGs with the fancy 300BLK id bands (like those stretchy bracelets, but tape is cheaper lol).

    Also FWIW, I seperate out my subsonic and supersonic 300BLK since I often use my 9mm can for subsonic 300BLK. Since the subsonic is for the fun times SBR 8 3″ and the supersonic for is the deer legal 16″ upper, I only buy tipped supersonic ammo for immediate ID to not use in the suppressor and the supersonic is stored completely separate ammo can and only goes in the 10 round hunting sand mag.

  • Max Glazer

    That is what I call a REAL flawed design. When you have a weapon that can lock its bolt and fire the weapon when a round is chambered with bullet being 2mm larger then the bore, then there is a REAL design flaw with that weapon. For example AK-74 will never chamber a round out of an AKM as round simply won’t enter the chamber, bolt won’t close and firing mechanism won’t perform action.

    • It generally can’t happen with most 300 BO ammo, but some bullet profiles may go. Also some with weak neck tension can be telescoped in.

      • Max Glazer

        As in the bullet will press itself deeper into the round? Please correct me if I’m wrong but there isn’t any free space inside the round that bullet can do into, is there?

        • Depends on the loading. This is more likely with handloads or with commercial loads made with components not designed around 300 BO, which do exist. i.e. using surplus or over run bullets such as the standard M2 Ball projectile from 7.62×51 Nato, or short light bullets intended for .30 carbine and similar. If the load uses a light bullet and gets it up to speed with a small volume of a fast pistol powder there could be a couple mm of space. Some hand loaders avoid crimping because they want to make brass last longer, so the bullet may have weaker neck tension. Further, if they are using a projectile made for other calibers, such as 30 carbine they crimping groove may just be in the wrong place relative to the neck. This all can add up to a bullet easier to push into the case.

          Then when the bullet gets seated into a .223 chamber, it can telescope in, compacting the powder a MM or two, which further exacerbates the pressure spike. Smokeless powder is usually not supposed to be compressed. It’s a complex subject, but generally speaking, when you compress smokeless it stops following predictable pressure curves.

  • kcshooter

    No reason not to have AR’s in multiple calibers, unless you aren’t smart enough to keep wrong ammo out of the wrong chamber.
    It isn’t the fault of round, or the gun, or the manufacturer, or anything else. Rather than trying to blame anyone else, take personal responsibility for you own actions.
    Yeah, it’s easy to mix up ammo – if you’re stupid.

  • JoelM

    If you make that many mistakes while handling firearms, maybe you should take it one step further from not owning a .300 Blackout and not own anything dangerous if misued. Sell your guns, cars, pointy objects. I’m surprised the Good Idea Fairy didn’t bring this up.

  • Eric

    There is no problem with 300 BLK, just people as usual…

  • John

    People should check out the Magazine Marker Bands on the Faxon site. They have .223/.556, .300blk, and 7.62×39.