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 makes it very popular among wasteland denizens, for whom sophisticated high maintenance weapons would be a liability to survival.
The 10mm pistol serves as the first decent firearm in both games, packing a little more punch than knives or fists. However, the 6520 becomes useless once 10mm MP9 SMGs start appearing, because the single shot mode of the MP9 is identical to the 6520's.