Fallout 3 crafting

Schematics allow you to create "self-made" weapons provided you have the proper ingrediants. They also have multiple copies located throughout the Capitol Wastelands allowing you to make better versions of the self-created weapons. Each copy you find past the first one allows you to make a better version (ie; Rock-It Launcher V2). So its a good idea to hunt down subsequent copies if you plan on using them often. Also higher repair will allow you to initally make stronger weapons. As you make multiple weapons you can combine them later on with the repair skill for even more damage. Alternatively you can have an NPC repair them.

List of Weapons with Schematics

 * Taken from Fallout 3 weapons.