Why You Need Clearly Marked, Designated Mags

The parent case of the 300BLK is 5.56. While you would think that a BLK cartridge couldn't chamber in a 5.56 rifle it can- at least enough to KB an upper receiver as pictured above at a range in Georgia. Fortunately no one was hurt- but this is important to understand and prevent because the next person may not be so lucky.
While the round probably didn't chamber, it got far enough that the bolt started to close and pushed the projectile backwards into the cartridge- basically smashing the charge and creating enough friction to set it off.

Scary? Yes but this is also easily avoidable:

  • Use separate, marked, dedicated mags for both BLK and 5.56
  • Some of the best ideas I've heard include:
  • use a different color mag for each caliber – like forest green for 5.56 and flat dark earth for BLK.
  • use 20rd mags for BLK and 30rd mags for 5.56
  • use mag sleeves for one caliber

Find a way to clearly mark your mags or make them so different that you can't screw it up- no one wants to be picking tiny slivers of aluminum out of their face.

4 thoughts on “Why You Need Clearly Marked, Designated Mags

  1. rbcadd

    A very good and important point to consider!

    My own strategy is to use metal magazines for the 300 AAC BLK rounds, and the plastic Magpul Pmags for the 5.56 rounds.

    Another reason to do this is I because have seen several different reports stating that the 300 AAC BLK tends to feed much more reliably from the metal G.I. STANAG magazines than from the Magpul plastic variety. It has something to do with the way the ribs that align and guide the cartridge as it moves toward the chamber are made.

    This way, I get the bast magazine for each round, and they are easily identifiable, even while groping in total darkness.

    I recommend using the ASC or AR-Stoner stainless steel mags for 300 AAC BLK. The AR-Stoner mags tend to be cheaper, at about $14 each from Midway, and they use the same ASC anti-tilt follower. I can’t really tell any difference between them, so I go for the AR-Stoner and save a few bucks.

    Notably, I also see that AR-Stoner has refused to engage in rampant price gouging during the current supply crisis, unlike many other manufacturers, so they deserve all the business we can give them. Now, more than ever, we should all really make it a point to daily reward people and companies that “do the right thing”, just because it is the right thing to do. The trend in the other direction has been going on long enough, and has gone far enough. It is now up to us to be the positive trendsetters and support other positive trendsetters.

  2. rbcadd

    Now we just need to come up with a way to quickly, easily, and positively identify each firearm, even in total darkness, otherwise the wrong magazine could still possibly find its way into the wrong firearm at a critical moment.

  3. k4R-15 Post author

    I’ve been thinking about this one for a few days. The best I have so far is to go with a distinctive hand guard type for different caliber rifles that take AR type mags…. Anyone have a better idea?

  4. rbcadd

    A note on magazine selection for 300 AAC BLK.

    As mentioned in my previous comment, I have seen reports stating that there can be feeding problems using the popular Magpul Pmag plastic magazines with the 300 AAC BLK round. Accordingly, I have bought some metal G.I. spec STANAG 5.56 NATO magazines to dedicate only to 300 AAC BLK. Reports are that these feed this round very reliably. It also helps me keep my rounds sorted out to avoid feeding the wrong gun with the wrong round.

    Many people have bought, or are considering buying, the aluminum Surefire 60 or 100 round magazine, designed for 5.56, to use for 300 AAC BLK, myself included. Some reports are glowing, indicating no feeding issues. However, I might beg to differ.

    While I have yet to actually fire any rounds from this magazine, I tried loading the Surefire 60 up to capacity and emptying it several times, and I ran into problems. Using a LULA device, loading went perfectly, but when emptying the magazine the big lower follower jammed and tilted severely, causing rounds to jam where the fat part of the magazine tapers into the neck. I cleared that jam, and, soon after, the smaller upper follower did the same thing, but in the neck of the magazine.

    I looked carefully inside and noticed that one of the two ribs that normally align and guide the 5.56 rounds, was now hitting the neck of the 300 AAC BLK round, while the other rides on the fatter part of the case, toward the case head, causing misalignment. For comparison, I closely examined my AR-Stoner stainless steel NATO STANAG magazines, and noticed that, in their design, the two ribs both ride on the fat part of the case, whether using 5.56 or 300 AAC BLK, as it should be. I have not seen any feeding problems whatsoever with these mags, with either round. This would be one major reason why.

    So, on that basis, I would advise that the Surefire 60 and 100 round magazines should probably be avoided for 300 AAC BLK use, unless and until Surefire re-designs their guide ribs to properly accommodate both 5.56 and 300 AAC BLK cartridges, as the AR-Stoner brand does, to avoid jamming of the latter during feeding and emptying.


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