Damage Resistance Modifier on ammunition in Fallout and Fallout 2, known as Penetration in Fallout Tactics, indicates how much to change the target's damage resistance. In Fallout and Fallout 2, a lower amount indicates higher damage. In Fallout Tactics, the opposite is true.

It does not work in Fallout due to engine bugs.

So for example, a 10mm JHP in Fallout 2 when examined reveals that it has a "DR mod" of 25. This means that against an enemy with DR 0, they would instead have DR 25 against any attack with that ammo. In this case, this ammunition also has a Damage Modifier of 2, which makes this ammo appropriate only for low-DR targets.

In Fallout Tactics, a hypothetical ammo with Penetration 25 would operate in the reverse: an enemy with DR 25 would instead have DR 0.

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