5: a supply chamber: as
a : a holder in or on a gun for cartridges to be fed into the gun chamber
b : a lightproof chamber for films or plates on a camera or for film on a motion-picture projector
Of course, it doesn't really matter what the dictionary definition is, it depends on how the courts interpret the law. Seeing as how it says "similar device", it could be anything. A clip, speedloader or revolver chamber.
Here's something weird- a Garand has an 8 round magazine, so is the Garand banned, too? They didn't say detachable magazine.