That magazine holds three rounds. For when you go to the range and want to shoot EXACTLY 3 rounds. No more. No less.
edit: OH wait that's the magazine well, yeah I'm a dumb dumb.
It might actually be a magazine, this other picture doesn't seem to have anything sticking out:
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but it's definitely not the normal magazine that would be used, that one would be (rather ridiculously long):
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"for when you go to the range and want to shoot EXACTLY <small-number> rounds" magazines do actually exist - they're meant for the early stage of development, when you're testing the purely mechanical aspects of the gun's accuracy (so, not the ergonomic aspect of how a real person aims it and controls the recoil, just "does the barrel stabilize the bullet enough", things like that, and also "will it not blow up when firing" if you happen to be developing a whole new cartridge), and will have the gun locked in some kind of vise, where a long magazine sticking out the bottom can be very inconvenient
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oh don't worry, i too first thought that was just a comically small magazine
