Fallout 3 crafting

Schematics allow you to create homemade weapons provided you have the proper components.

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). It's a good idea to hunt down additional copies if you plan on using the weapons often.

Higher repair will allow you to potentially make homemade weapons that have higher condition and damage. As you make multiple copies of the same weapon you can combine them with the repair skill for higher condition and damage. Alternatively you can have an NPC repair them.