Download the GURPS High-Tech Pulp Guns sourcebooks. It's loaded with all sorts of period firearms from the late 19th century up to the 1930s, including weird ones like the HDH Mitrailleuse (basically that revolver up there, but with only 2 barrels).
I always have trouble finding it, but there's a website detailing pocket pistols (mostly early 20th century ones) and there's a .25 ACP pistol where you don't take the magazine out after the gun is empty. You just shove the new mag straight into the magwell and it pushes the spent one out the ejection port. I can think of a few ways that could screw up, but it would be a neat trick for a sci-fi pistol.
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