Accuracy AC and Impact AC (3.5e Variant Rule)
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 Accuracy AC and Impact AC
After playing D&D for some time, it becomes clear that a character's single AC score is composed of elements that represent one of two completely different things. One thing represented by AC is how hard it is to hit a character--bonuses such as dexterity, dodge, and insight reflect this. But AC also represents another, totally different thing. Bonuses such as armor and natural armor assume that a character has already BEEN hit, and the AC is being used to determine whether or not the hit actually penetrates the armor.
These two things are quite different. And yet the single AC implies that they are the same, implying laughable notions such as agility and heavy armor protecting a person in exactly the same way, and a high Strength score making it easier for a barbarian to hit a monk who relies entirely on Dexterity and Dodge bonuses for his AC. Or that a ballista is equally effective against an unarmored but Tiny and agile creature as it is against a slow, hulking, but well-armored Huge creature. Or, the most infamous example of all, that an ordinary house cat stands a decent chance of killing a human commoner in a one-one-one fight.
This variant rule is designed to add realism, as well as, to a lesser extent, strategy to the game by creating two wholly distinct AC scores and respective attack roles. One, termed the Accuracy AC score and attack roll, determines whether an attack hits its target. The second, termed the Impact AC score and attack roll, determines whether an attack that already HAS hit its target will actually cause any serious injury (as opposed to just a minor scratch, cut, or bruise).
Warhammer 40,000 players will be somewhat familiar with this mechanic: the Accuracy roll is comparable to a "roll to hit", and the Impact roll is comparable to a "roll to wound".
 AC Scores
Under this variant rule, a character has two separate AC scores: an Accuracy AC score, representing how hard he is to hit, and an Impact AC score, representing how hard he is to injure once he's hit.
The Accuracy AC for a character is determined by the following formula:
Accuracy AC = 8 + Dex bonus + Shield bonus + Size bonus + Deflection bonus + Dodge bonus + Insight bonus + Luck bonus + Cover bonus
The Impact AC for a character is determined by the following formula:
Impact AC = 2 + Armor bonus + Natural Armor bonus + Deflection bonus + Luck bonus
The specific bonuses are determined in the same way as under the normal SRD system.
 Base Attack Bonus
Under this variant rule, a character also has two separate BAB scores: an Accuracy BAB score, representing how good his aim is, and an Impact BAB score, representing how good he is at striking hard and finding weak spots in armor.
Whenever a character would gain an increase in BAB from a new level, he may choose to either increase his Accuracy BAB or his Impact BAB. For instance, a level 10 Fighter might choose to have an Accuracy BAB of 4 and an Impact BAB of 6, an Accuracy BAB of 9 and an Impact BAB of 1, or an Accuracy BAB of 2 and an Impact BAB of 8.
Regardless of how a character chooses to allocate his BAB scores, both Accuracy and Impact BAB are added together for the purposes of qualifying for feats and determining extra attacks for a full-round attack.
 Attack Rolls
For most attacks, two separate attack rolls will be made. The first roll is a character's Accuracy attack opposed by the target's Accuracy AC—a roll to determine whether an attack hits. If the attack roll beats the AC, a second roll is made, determined by the attacker's Impact attack opposed by the targets Impact AC—a roll to determine whether the hit causes serious injury.
Accuracy Roll (for all weapons): 1d12 + Accuracy BAB + Dex modifier + Size modifier + Insight bonuses + Morale bonuses + Luck bonuses + Special attack modifiers (flaking, prone defender, high ground, etc.) − Two-Weapon Fighting/Flurry of Blows penalty − Power Attack feat use
Impact Roll (for melee weapons, composite bows, slings, and thrown weapons): 1d8 + Impact BAB + Strength modifier + Morale bonuses + Luck bonuses + Magical weapon enhancement bonus − Two-Weapon Fighting/Flurry of Blows penalty − Combat Expertise feat use − Fighting Defensively penalty
Impact Roll (for crossbows and non-composite bows): 1d8 + Impact BAB + Luck bonuses + Magical weapon enhancement bonuses + Weapon's special Impact modifier + Weapon size modifier − Two-Weapon Fighting/Flurry of Blows penalty − Combat Expertise feat use − Fighting Defensively penalty
Special Impact Modifier for crossbows and non-composite bows:
Hand Crossbow: +2
Light Crossbow: +4
Heavy Crossbow: +8
(Just as in real life, a crossbow's main claim to fame is its ability to punch through armor like it was wet tissue paper)
Weapon Size Modifier for crossbows and non-composite bows: Crossbows and non-composite bows sized for Large creatures have better Impact than similar weapons sized for Small creatures. The Weapon Size Modifiers for crossbows and non-composite bows are thus:
A target takes Hit Point damage if and only if both the Accuracy and Impact attack rolls are successful.
The rules of how Strength modifiers are applied to light weapons, one-handed weapons, and two-handed weapons remain unchanged.
 Automatic Successes and Failures
On Accuracy rolls, a 1 is an automatic failure and a 12 is an automatic success.
On Impact rolls, there are no automatic successes and failures. Sorry, but I just can't see how firing a sling at a great wyrm is supposed to have a one in twenty chance of injuring it. You'd better make sure your attacks are strong enough to penetrate the target's armor, or else you're in trouble. There will be no Ewoks defeating Stormtroopers by chucking rocks at them under this variant rule, no sir. Similarly, I fail to see how the aforementioned great wyrm could possibly fail an Impact roll against an unarmored commoner.
 Critical Hits
Critical hits under this mechanic work in more or less the same way as they did before. After both the Accuracy roll and the Impact roll have been confirmed as "hits", you may roll for a critical. Roll a d20 as usual, and if the roll falls within your weapon's threat range, that's a critical threat. To confirm the critical, make one final Accuracy attack roll.
 Touch Attacks
For touch attacks, no Impact roll is needed. Just roll for Accuracy.
 Weapon Finesse
Under this variant rule, the Weapon Finesse feat no longer does what it used to. Since the Dex modifier is applied to ALL Accuracy rolls, it's original function is now obsolete.
Instead, Weapon Finesse now does the following: After declaring an attack, but before making any rolls, you may choose to sacrifice an amount from your Accuracy roll of up to your Dex modifier, and add it to your Impact roll.
The rule of what weapons Weapon Finesse can be applied to (light weapons and special weapons such as rapiers) remains unchanged.
In game terms, the feat represents the ability to use light weapons to exploit gaps, flaws, and soft spots in a target's armor.
 Range Increment Penalties
Use your imagination. Some would argue that this is strictly an Accuracy penalty. Others might argue that since a projectile loses momentum and angle the longer it travels, it should be an Impact penalty. Still others would say that when you factor in perspective and the fact that an arrow to the pinky finger isn't exactly a life-threatening injury, or that it's difficult to locate weak spots in the armor of someone 500 feet away, that range increment penalties should apply to both Accuracy AND impact. Your DM is ultimately the final word in this debate.