The World War I-era U.S. Army was unprepared for fighting a global confrontation in the 20th Century. Hell, it was unprepared for any modern confrontation at the turn of the century. As America prepared to enter the Great War, the War Department called on its military minds to develop a lightweight, short-range, trench-clearing game changer. The result was the Thompson submachine gun. The "Tommy Gun," as it came to be called, used the Colt M1911 grip and its dependable .45-caliber ammunition. By 1919, the fully-automatic weapon was perfected, and it was capable of using a 20-round block magazine or a 50- to 100-round drum magazine. But the war was over and the surplus was sold on the civilian market to anyone who could afford one – including notorious gangsters.
It was the outlaws and gangsters who made the Tommy Gun iconic.
In nearly every photo of the era, the gangsters can be seen using the drum magazines, which provided them more ammunition for the weapon's high rate of fire. It makes sense for an outlaw to use more ammo when trying to make a quick, clean getaway from the fuzz. Shouldn't it make sense for U.S. troops to do the same when advancing in World War II?
The answer is no, and not just because a 100-round magazine will help deplete ammunition much faster than having to conserve 20- or 30-round box mags. It turns out, the Thompson was really bulky and not so easy to carry while slung with a drum magazine. More than just being unwieldy, the rounds tended to rattle inside the drum magazine and produced a lot of unwanted noise, noise that could get an entire unit killed in combat.
But the most important reason was reloading.
Switching between a drum magazine and a box magazine required an extra set of tools. To load a drum magazine also required the user to have a special tool that would lock the bolt back to the rear. And, unlike spring-loaded box mags that were already under tension, reloading a drum magazine required a tool to rotate the spring in the magazine enough to put the rounds under the necessary tension.
Worst of all, if you lost any of the tools needed to reload the weapon, you would be hard-pressed to actually be able to do it without assistance. Drum mags also weighed more and took up more space in a very limited kit. Whereas the box magazine could be loaded and dropped from the rifle in seconds, shared with a buddy, and reloaded just as fast.
The difference between 30 second and 3 seconds under fire in World War II could have been the difference between life and death. In gangland Chicago, all you needed was time for your V8 Packard to speed away before the Untouchables swooped in.