It is the most common type of ammunition in the Capital Wasteland, with the second most common being the .32 caliber round. The weapons that use such ammunition are also common, and there are many different types of guns with different statistics that use them, making 10mm rounds very useful. Made by G, it comes in ammunition boxes of 12 rounds.
Three boxes in the Super-Duper Mart, on the counter behind the terminal opening the storage room.
The Lone Wanderer is given a 10mm pistol, along with 36 rounds for it, if the right speech options are chosen during the quest Escape!
The Pip-Boy icon for 10mm rounds and the ammo clip model loaded into the Chinese pistol depict 10mm rounds as bottlenecked cartridges, but other game's appearances of the ammunition as well as the magazine textures for the 10mm submachine gun show the cartridges as having a straight case.