Fallout 3 crafting

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Multiple copies of a schematic
Obtaining more than one copy of a schematic upgrades the plans. Additional schematics cease to yield benefits after three of each kind have been located; they will simply appear as items in your inventory.

For non-explosives, two plans increase the initial condition of the weapon at construction to 100% of your repair skill (from 75% of your repair skill with just one blueprint). Three plans net a condition of 125% of your repair skill at construction; however, you cannot build a weapon with a higher starting condition than 100%.

For the bottlecap mine and Nuka-grenade, additional schematics allow you to make more items from a single set of components instead. With one schematic - you make one. With two - you make two. With three - you make three.

Additional schematics do not increase the base damage of a weapon. As the base item is the same, the base statistics are as well. The change is in its starting condition (or number, for the bottlecap mine and Nuka-grenade), which does increase the effective damage of the weapon.

Repair skill
Higher repair allows you to make homemade weapons that have higher starting 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 non-player character repair them. The bottlecap mine and Nuka-grenade are always at maximum condition.