100-Round AR-15 Double Drum Magazines for Dummies


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.