This is a Walther P-38 magazine, which is called a single-stack magazine because that's exactly what's going on. There's a single column of cartridges, one on top of another, feeding out the top.

These are not sapphire bullets of pure love, they're just mildly artsy dummy rounds.

This has the advantages of relative simplicity (for a detachable magazine) and compactness. The frames, and thus the grips, of single-stack auto pistols can be slimmer, and are often considered more comfortable as a result.

Compare that with the magazine from a Browning Hi-Power:

This is a double-stack magazine. As you can see, there are two columns of cartridges in there, somewhat overlapping to make the best use of the space. The advantage of this arrangement is that it can fit more ammunition into the same vertical distance, which in handguns means that given two pistols of comparable size, the one with the double-stack magazine will have a higher capacity. The downsides are that they're a little more complicated to make and they tend to make the grips of the pistols they're fitted to a bit fatter, which some shooters (particularly those with smaller hands) find less comfortable.

Technically, what the Hi-Power here has is a double-stack single-feed magazine—you can see that it has a taper at the top so that it only presents one cartridge in the center. It is possible to design a firearm that can feed from either side instead (double-stack double-feed); a number of modern self-loading rifles and SMGs work that way, but as far as I know it's still pretty rare in handguns.

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
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