For an overview of 10mm pistol models, see 10mm pistol.
“A Colt 6520 10mm autoloading pistol. Each pull of the trigger will automatically reload the firearm until the magazine is empty. Single shot only, using the powerful 10mm round.”— Fallout and Fallout 2 description
Standard-issue for Vault 13 security, the 6520 is a highly durable and efficient weapon developed by Colt Firearms prior to the Great War. It proved to be resistant to the desert-like conditions of the post-nuclear wasteland and is a fine example of workmanship and quality construction (at least for the pre-War models, which became more scarce over time). The simple design of the gun allows it to be easily manufactured with relatively basic equipment, making it possible to be replicated in workshops in the post-nuclear world.
The 10mm pistol's visual appearance is taken directly from the comic book Hard Boiled. Geofrey Darrow, the artist for Hard Boiled, would later go to illustrate the cover art for All Roads, which was released in conjunction with Fallout: New Vegas.
10mm pistol being an autoloader is a result of miscommunication between the developers. 10mm pistol was intended to be a revolver.[Non-game 1]
↑Chris Taylor in Fallout Bible 8: "The longer answer: Miscommunication. I think the text was written before the art was done, or the art was redone and the text wasn't updated (it's a little fuzzy). Should be "...until the cylinder is empty." But I'd probably want to rewrite the description completely."