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
It 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 Colt 6520's durability, reliability, and simple design make it very popular among wasteland denizens, for whom sophisticated high maintenance weapons would be a liability to survival.