100-Round AR-15 Double Drum Magazines for Dummies

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According to the latest info on the midnight movie massacre in Aurora, CO, spree killer James Holmes opened-up on the crowd with a Remington 870. After emptying the shotgun, he switched to a Smith & Wesson AR-15 equipped with a “double-drum” 100-round magazine. At some point during his heinous crime, the mag jammed. Holmes didn’t know how to clear his weapon. So he abandoned the rifle and switched to a Glock .40 caliber pistol. Police aren’t providing a total round count for the entire attack, but there’s no getting around it: the 100-round drum magazine’s failure saved lives. No surprise there. Double-drum magazines suck. Here’s why . . .

1. Double-drum magazines jam  

Most semi-automatic AR-style (a.k.a. “assault”) rifles come equipped with 30-round magazines (except in states which already have a “high capacity” magazine ban, which limit owners to 10-round magazines). It’s a practical limitation. There’s a point after which magazine reliability decreases as the stored round count increases. Dramatically. One hundred rounds is well beyond this practical and literal point of no return.

Even so, aftermarket manufacturers sell 100-round magazines for AR-style rifles. The $179 Surefire magazine is (to my knowledge) the most reliable of the breed. It’s NOT a double-drum-style magazine. Double-drums, as above, are notorious for jamming. Two separate springs have to apply an equal and even amount of pressure at the same time to assure reliable feeding. Introduce other variables—manufacturing errors, temperature fluctuations, gun movement, minor differences in bullet size, etc.—and the cartridges can fail to make it round the horn and into the gun.

It’s only a matter of time before a double drum mag fails, producing one of at least two different problems (e.g., failure to feed and double-feed). While there are ways to address AR-15 magazine failures, there’s no guarantee they’ll work or, if they do, that the shooter will be able to continue shooting unimpeded by another or ongoing mechanical failure. If a criminal wants a reliable way to deliver 100-rounds from a rifle, a double-drum magazine is the worst possible choice. Thank God Holmes chose it.

2. Double-drum magazines limit accuracy  

In the video above, the shooter is firing his double-drum-equipped AR from a seated position with his support arm resting on the bench and his support hand on a fore grip (the handle on the front of the gun). While we don’t see his target, that’s about as good as it gets.

If you’re holding an AR-15 with a double-drum magazine “free hand” (no support) you are holding one heavy gun. One-hundred rounds of .223 ammo (the kind Holmes used in his AR) weighs around three pounds. A Smith & Wesson AR-15 weighs a little under seven pounds. So call it ten pounds total.

While adrenalin makes heavy things seem lighter, accurately aiming a ten pound gun for an extended period of time is no easy task. As the rounds empty, as the gun gets lighter, the handling changes. At the same time double-drums extend the width of the AR, forcing the shooter to “chicken wing” his support arm’s elbow, further reducing accuracy.

We don’t know Holmes’ “hit ratio” but we do know a local gun club turned down his application. If he’d had more experience or chose a better handling AR, one without a double-drum magazine, he would have been more accurate. Perish the thought.

3. Double-drum magazines are hard to conceal

I wish I could say that concealability issues limited John Holmes’ attack. They didn’t. But that doesn’t change the fact that the vast majority of criminals use handguns in gun crimes. Why wouldn’t they? They’re easier to hide. And if handguns are easier to hide than rifles, a rifle equipped with a double-drum magazine is twice as obvious as one without. A rifle with a double-drum magazine would draw more attention than one with a standard mag, should someone see a criminal carrying an AR so equipped.

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About Robert Farago

Robert Farago is the Publisher of The Truth About Guns (TTAG). He started the site to explore the ethics, morality, business, politics, culture, technology, practice, strategy, dangers and fun of guns.

24 Responses to 100-Round AR-15 Double Drum Magazines for Dummies

  1. avatarjwm says:

    that setup looks clumsy. i’ve nuver used a drum magazine in an ar but as a youngster my uncle had a tommy gun with stick and fifty round drum. world of difference when you switched to the drum. from my own experience with the m16 just changing from 20 round mags to 30 rounders made a small difference in how the weapon handled. for a newbie like holmes with little or no training it made a huge difference and likely did save lives.

  2. avatarMatt in FL says:

    The round count is the one remaining piece of information I’m interested in about this case. I don’t know what I’ll do with it once I have it, but I’m curious all the same.

  3. avatarSkinnedknuckles says:

    “he switched to a Smith & Wesson AR-15 equipped with a “double-drum” 100-round magazine. At some point during his heinous crime, the mag jammed. ”

    How does this square with the picture of the abandoned AR on the ground outside the door of the cinema that had a convention magazine mounted?

    • avatarjwm says:

      what’s the context of the picture. that could be an leo weapon set aside for a moment while they’re cuffing the suspect or helping a victim. we don’t know if that was the shooter’s weapon.

      • avatarSkinnedknuckles says:

        http://www.dailymail.co.uk/news/article-2176377/James-Holmes-Colorado-shooting-Gunman-used-drugs-killed-Heath-Ledger.html

        You can view the pictures and make your own decision, but there appears to be an evidence marker on the rifle.

        • avatarjwm says:

          s.k. you’re right, they aren’t the best photos but it does look like an evidence marker next to the rifle. it would be worse than negligent if an leo laid his own weapon down amongst the evidence like that. i’m giving them the benefit of the doubt and will say that;s the “jokers” weapon with 30 round mag. so he must have done at least 1 mag change or he never used the drum unit. odd isn’t it? but news sources are famous for not getting it right, maybe he never used the drum.

        • avatarmatt says:

          He must of. Otherwise why would he have ordered a tactical vest that was designed to be used with STANAG magzines?

        • avatarHSR47 says:

          If I had to guess, it seems probable that he had a double feed and assumed he had run the mag dry, thus he reloaded with a 30 rounder.

        • avatarإبليس says:

          So why did to Daily Mail call it a Colt instead of S&W? General ignorance or a second unused rifle or what?

      • avatarPascal says:

        All the news stories I read until the press started to twist things said that the drum was found at the scene but nobody actually said it had been used. He could have gotten a jam with the 30 round magazine.

        IMHO, the media is hyping the 100 round mag but I am willing to bet it was not used except for propoganda to have an AW Ban

        • avatarMatt in FL says:

          And that’s precisely why I said (below) that magazine conjecture is best left for when the report comes out. Before that, it’s just spinning wheels.

    • avatarMatt says:

      I was wondering the same thing! The photo was taken durning daylight at the crime scene. Right outside the door of the theater there is an AR-15 laying there with a 30 round magazine in it, plain as day. Something is fishy about that. There is a forensics team taking photos and gathering evidence so this weapon is not a LEO weapon casually laid to the side. This weapon must be the one used but there is no 100 round drum magazine. You can find the photo if you search google images although it may take awhile. I noticed this image on a news website the day the shooting occurred before they were talking about the 100 round mag.

    • avatarMatt in FL says:

      Responding to all above: I think the magazine conjecture is best left for when the report comes out. Perhaps when the 100-rounder jammed, he dropped it, swapped in a conventional 30-round, and shot it dry (or not). Remember that the first reports only said that a 100-round drum mag was found “at the scene.” Some commented at that point that it may not have even been used, but was in his car, which would still be “at the scene.” Now it’s clear it was used, and that it jammed, but we don’t know at what point that occurred or what happened afterwards.

      • avatarSkinnedknuckles says:

        I guess I’m not even sure we really know that it was used or that it jammed. With the gag order facts are few and far between (and I wish I could say the same thing about “news stories” about the tragedy). It will be a long time before enough facts become public to start to piece together what actually happened, just like Florida Our children will get to read Mas Ayoob’s analysis.

  4. avatarmatt says:

    2. Double-drum magazines limit accuracy
    Do you really need to be accurate when shooting in to a crowd of people in a enclosed space? And it would be more accurate to say high capacity magazines limit accuracy, can you imagine shooting from a bench with a 100rd Surefire mag?

    3. Double-drum magazines are hard to conceal
    It would be more accurate to say, high capacity magazines are hard to conceal. Have you seen what a AR looks like with a 100rd Surefire mag?

    I’m surprised he went with the Beta magazine, it costs about 50% more than a Surefire.

    And according to Wikipedia “In November 2008 the Army Experimental Task Force (AETF) at Ft. Bliss, TX, evaluated six BETA C-MAG magazines. Four magazines—two with black covers and two with clear covers—were used with M4 carbines in three firing scenarios: controlled pair, controlled burst, and rapid fire. According to the memorandum summarizing the evaluation, the four magazines “performed flawlessly in all three scenarios without jams or stoppages.” In addition, two magazines with black covers were evaluated with M249 light machine guns in controlled burst and rapid-fire scenarios. These also performed without “issues,” according to the memo, which also notes that soldiers “had only positive comments” about the C-MAG magazines during the After Action Review (AAR).[5]“

    • avatarmatt says:

      Not that i’ve ever used one but Beta Co. seems to blame jam issues on a lack of graphite lubrication of magazine. If they were as problematic as people like to think, you’d think they would have gone out of business a while ago.

  5. avatarmdc says:

    I got one thing to say.1997 Bank of America robbery gone bat shit crazy.And look what was used and time tested.AR’s have there place and so do AK’s.

  6. avatarjanklow says:

    well, i’ll be the one to say it: there is also the possibility that one’s state bans high-capacity magazines but allows more than 10-round magazines to be sold. strange but true.

  7. avatarAnotherMatt says:

    Exempted from this, of course, are the excellent 75 round RPK drum magazines. Those Russians sure know how to make a good magazine.

  8. avatarDex says:

    drum magazines for dummies?

    word of advice: stay away from them.

  9. avatarCharlie says:

    I always thought drum mags were for the hollywood crowd (sorry). Beside the reliability issue they’re incredibly bulky, and I can only imagine the stress that 3 lb of inertia is delivering to the drum’s throat under recoil.

    There is a practical limit to mag size and weight, and this double drum crud is way over it.

    Charlie

  10. avatarJonathan Miller says:

    I shouldn’t believe a word in The Daily Mail, a newspaper that has a troubled relationship with facts.

  11. avatarEd the Fed says:

    I own two 100 rd AR drum mags made in Korea by KCI. They are virtuallly identical to true Beta mags and mine feed/function perfectly. Just very heavy and not practical. Fun though!

  12. I got mine on Sale 2 moths ago from Centerfire Systems for $100. It works well. It is worth it to own just in order to piss off the gun-grabbers.

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