Remove the <li>FCP_Gun_32_Pistol</li> entry from the Megaton trader's thingDefs list in 1.6/Defs/TraderKindDefs/TraderKindDefs_Megaton_Shops.xml so the vendor no longer offers that weapon.
Add Megaton settlement support and inventories. FCP_Wastelander_Factions.xml was updated to register a named settlement 'Megaton' with five traders (Craterside Supply, Brass Lantern, Megaton Clinic, Moriarty's Saloon, Water Treatment). A new TraderKindDefs_Megaton_Shops.xml defines the corresponding TraderKindDef entries and detailed stockGenerators for each shop (resources, food, weapons, apparel, medical chems, drugs, building materials, buying behavior, and optional MayRequire blocks for modular content). This wires up in-game shops and their stock for the Megaton settlement.
RickGrymes