Thursday, June 27, 2013

2d20 game mechanics for warfare RPGs

This post puts forward a preview of the game mechanics used in a modern combat RPG I'm working on. These game mechanics are an evolution of the mechanics used in Itza and Imperium & Maleficium, prehispanic and medieval fantasy RPGs. They focus on solving certain issues that I see arise with modern weapons and tactics.

Whereas combat in medieval settings is more a one to one thing, modern warfare with their rate of fire and range turn combat into a one to many engagement. Your character's firepower is such that the classic, one attack roll to hit falls short of the required dynamics.

To overcome this I've written up some new rules that address issues with initiative, movement, rate of fire and area coverage.

Complex modifiers
- Movement : Movement is life on the battlefield. Movement is not seen as an action, instead it is a modifier to actions. Movement is something your character does as it performs actions, it adds fatigue and drops the fire precision.
- Rate of fire : Rate of fire is a modifier to hit. Putting a lot of fire in an area is a means to increase the odds of hitting hidden or unknown targets in it.
- Aim : Aiming a weapon increases the odds of putting a bullet on the target.
- Cover and Visibility: Cover decreases the odds of being hit by a bullet. Even cover that is not solid makes the soldier harder to see and thus harder to shoot at. This requires the shooter to spread the fire into an area and thus reduce the odds of hitting the target.

Complex activities
- Many attacks (actions) per round : The amount of actions a character can take during a round are not a fixed arbitrary number. They are variable and depend on the amount of actions the player wants to take (to a certain limit set by the character's dexterity). The more actions the character takes, the higher the fatigue and the greater the penalties that will be suffered on future rounds.
- Suppression and cover fire : This is a type of fire applied to an area with the intention of not letting the enemy in the area fire back. Hitting the enemy is a goal, but the main objective is psychological, stopping the enemy from taking action and enabling friendly units to move into a more favorable position to engage the enemy.

2d20 abridged rules

Combat Round

The combat round is a ten second span of time during which actions are taken. The amount of actions a character can take depend on the character's movement rate and endurance. Actions add heart load to the character, the more actions the higher the heart rate and the more prone the character is of running out of breath. The end of a round signals the recovery of fatigue, but not necessarily the end of the attack or action the character is currently undertaking. A round is simply a unit of time to measure character fatigue. The start of a new round may or may not require an initiative roll.


Initiative is a check against the character's mettle attribute. This is performed by rolling 2d10 and adding. The result must be equal or less than the character's mettle attribute. The better the roll the cooler the character is during the round and the quicker the response to combat events.

Initiative is not necessarily rolled at the start of every round, instead it is rolled when a confrontation starts. A confrontation may span over the time frame of a round or more.


Movement is the activity that sets the base fatigue level for the character. Fatigue works based on the character's heart rate. The faster the character moves the higher the base heart rate will be and the less actions it will have available before running out of breath.

Movement also affects combat. A character that's moving will be able to aim less, but at the same time will become a harder target to hit. Unless of course the character runs straight into the firing gun.


Attacks are resolved by two 2d20 rolls, one for the attacker and one for the defender. The dice on each roll are added and compared. A hit is scored if the difference (called delta) between the rolls is less than 10 plus modifiers (called delta t, t for target).

For example:
Attacker rolls 2d20 : 4, 8 = 12
Defender rolls 2d20 : 11, 4 = 15

The attack hits since the delta = 15 - 12 = 3 which is less than 10 (delta t).

delta - the difference between the attack roll and the defense roll
delta t - the required difference to score a hit

Delta must be less than or equal to delta t to score a hit.


Both delta and delta t can be adjusted by various modifiers. Delta is adjusted by aiming and weapon precision, in general any activity that increases or decreases weapon precision. Delta t is adjusted by target movement, rate of fire, cover and visibility, in general any activity that increases or decreases character exposure.

As an attacker you want a high delta t, delta t means the odds of the target getting hit. Modifiers that add to delta t are good for you as an attacker. Modifiers that affect delta are good if they reduce delta, delta means the precision of your bullet. The smaller the delta the more precise the shot.

Delta modifiers


Aiming decreases delta by making the shot more precise.
Aiming a weapon subtracts 2 to delta
Aiming a weapon with a scope subtracts 3 to delta
Aiming a weapon with a high power scope subtracts 4 to delta

Shooter movement

Moving increases delta by making the shot less precise.
Walking adds 2 to delta
Running adds 4 to delta
Sprinting adds 6 to delta


Range increases delta making the shot less precise the further the target is.
Weapons will provide a range modifier to delta.

Delta t modifiers

Rate of Fire

Weapons will provide a delta t modifier for different rates of fire. Higher rates of fire put more bullets in an area and increase the odds for a hit. This increases the exposure of the units under fire. Rate of fire increases delta t and makes the target more probable to hit, a good thing as an attacker. Although it may also add some aiming penalties due to recoil.


Character visibility will reduce delta t by making the target more difficult to hit.
Modifiers are as follows:
Clearly visible : -0
Vaguely visible : -2
Hardly visible : -4

Cover and Shields

Both cover and shields provide a delta c (c for cover). Any attack that hits (delta less than or equal to delta t) but who's delta is greater than or equal to delta c hits the cover or shield first instead of the character. The hardness of the cover or shield will determine if the attack hits the character or not. See damage roll below.

For example a soldier is behind a brick wall. This gives him a delta c of 4. The soldier already has a delta t of 10, so any attack roll with a delta of 10 or less will hit the target, but any attack with a delta between 4 and 10 actually hits the wall in front of the target. The wall's hardness will determine if the bullet is stopped or not. The GM rolls for bullet damage and wall soak as if it were normal damage. See damage below. A bullet that exceeds the cover's soak roll will continue onto the target with the remaining momentum. It will then try to go through the target's armor and body.

Armor and Damage

Armor (shields and cover included) have potential to absorb momentum from the ammunition. This is represented by a armor soak roll. The damage of the ammunition is represented by a damage roll. When a character takes a hit a soak and damage roll are made. Each die on the damage roll is compared to a die on the soak roll. Dice are not added, but rather compared in descending order. A damage die does damage if the value is greater than the soak roll. All damage rolls that exceed their corresponding soak rolls are added to obtain the total damage.

If the ammunition hits a shield or cover first the shot must first roll against the shield's or cover's soak and then against the body armor if any.

Any damage that penetrates body armor will damage the character. Characters have hit points and stamina points. Stamina works as a damper for damage. The soldier's pain threshold is an amount of stamina points that are deducted from the weapons damage on every hit. If there are still damage points after subtracting the pain threshold value these points are done as damage to the character's hit points. For example a character with a 5 pain threshold takes 8 points of damage from a bullet. The stamina takes 5 of these points and three pass through to the character's hit points.

Cover Fire

Cover fire is the action of putting a lot of fire power on an area with the intention of not letting the enemy fire back. Cover fire provides a delta t modifier for shock (delta s). Any attack who's delta falls above delta t (considered a miss), but less than or equal to delta s has the potential to shock and scare the soldier. A mettle check must be done to remain cool. For example a weapon, given it's ammunition type has a delta s of 4. A soldier fires upon an entrenched enemy unit. The roll gives a delta of 12, not enough to beat the required delta t of 10, but it's still below 14 (delta t + delta s, 10 + 4 = 14). No enemy is actually hit but they're all intimidated and take cover.

Mortar Fire

Mortar rounds have an area of effect. Mortar rounds will hit a spot and those around it will be affected by the blast. The mortar gets a 2d20 roll and everyone in the blast radius gets a 2d20 roll. The mortar round applies delta t modifiers depending on distance from blast.

Blast radius:

60mm HE - lethal 20 m - modifiers : 10 m +2 delta t
81mm HE - lethal 34 m - modifiers : 15 m +2 delta t
107mm HE - lethal 40 m - modifiers : 20 m +2 delta t
120mm HE - lethal 60 m - modifiers : 20 m +4 delta t, 40 m +2 delta t


A Marine squad is patrolling through the forest on a search and attack mission. The point man comes under fire. The GM rolls for the attacker's AK-47 (10 + 17 = 27), they have automatic initiative due to ambush.  The player rolls for the point man (20 + 16 = 36), delta = 9 (36 - 27) the attack hits (delta <= 10).

The GM rolls damage for the AK-47 (3d12 : 10, 8, 3). The player rolls for the body armor (3d10: 8, 8, 4). Comparing one by one in descending order: 10 vs 8, 10 wins; 8 vs 8, tie; 3 vs 4; 4 wins. The armor stops two of the three damage dice. The 10 passes through and does 10 hit points of damage. The point man's pain threshold is 4, this takes 4 stamina points away and 6 hit points of damage are done to the character. Since this is more than 25% of the total hit points the player must roll and endurance check (2d10 vs 13 endurance). A 14 is rolled, the character fails the check and falls to the ground. He's still alive, but unconscious.

Behind the fallen Marine the M249 lights up. The character operating it yells, "Contact, to the left, high, by the rocks", as he moves to take cover behind a tree. The man behind him throws a smoke grenade at the rocks. The last member of the fire team opens up with his M4 on the rocks. The fire team comes under fire again as it moves to cover.

As the team moves to cover they enjoy a -2 to their delta t. Small delta t is good as a defender, bad as an attacker. The enemy rolls 2d20 : 8, 29, and 16 vs the players 32, 38, and 22. The corresponding deltas are 24, 9, 6. Since they're a harder target to hit (-2 to delta), values have to be 8 or lower to obtain a hit. Only one of the soldiers is hit.  The damage is rolled (3d12 : 7, 6, 6) vs (3d10, 9, 7, 5). Only the last 6 beats the armor's 5, all other rolls are soaked by the armor. The character suffers 6 points of damage, 4 go to stamina loss and 2 create a slight flesh wound (less than 25% of total hit points). The slightly injured soldier makes it to cover.

The GM rolls 2d20 for the smoke grenade position (10) and the player 2d20 for the shot itself (16). The delta is less than 10 so the grenade lands pretty much were it was needed and will begin to block the vision.

Now we resolve the machine gun fire. The player with the machine gun (M249) rolls 24 vs the GM's roll of 29, 21 and 23 for the attacking enemies (the GM rolls secretly so not to reveal enemy numbers). To determine if the attack hits the difference between the rolls (delta) must be less than delta t (delta to hit) after modifiers ( 10 + modifiers). Any roll who's difference is less than or equal to this new modified value is a hit.

The M249 has the following modifiers:

  • to delta t -4 for low visibility (visibility modifier, bad for the attacking Marine, good for the enemy)
  • to delta t +2 for automatic fire (rate of fire modifier, good for the attacking Marine, bad for the enemy) 
  • to delta +6 for moving while firing, (bad for the Marine, good for the enemy)
  • delta s +3 (delta shock), any roll that does not hit, but is up to 4 above delta t causes fear

Total delta t modifiers is -2. The required delta to hit is 8 or less (10 - 4 + 2= 8, base delta t plus modifier).

The total delta modifier is +6.

The delta for the rolls are : 5, 3 and 1.

After applying the +6 delta modifiers the rolls become 11, 9, 7. The first two are misses since they're above the required delta t of 8 to hit, but they're still in the shock margin (11, 8 + 3 (delta s)). The delta s is 3 so any delta less than or equal to 11 (delta t + delta s = 6 + 3 = 9) causes fear. The first two soldiers fall prey to suppression. The last one takes a hit from the M 249 and dies (damage and soak rolls omitted).

The GM dictates that enough time has passed to signal the end or the round (10 seconds). The next round begins, characters add up fatigue (omitted for clarity) and start a new round.

Fire team A has turned the ambush around and is now pinning the assaulting group behind the rocks. Enemy strength is unknown. The point man is down and a medic is coming up to aid alongside fire team B. Fire team C has moved to the trees and is now looking for more hostiles uphill.

Fire team C sees movement along the rock uphill and yells "Contact!". The players roll initiative using 2d10 against their mettle attribute. They have values between 14 and 15 so they need to roll below that. Two of the team members succeed with the check, one fails and the team leader succeeds exceptionally well by rolling a 7. He's in total control of the situation and notices that the enemy has not seen them and is putting their attention on fire team B and the medic taking care of the casualty. He signals his men to take aim with their rifles and wait for his command. As they enemy sets their aim on Fire team B he orders them to fire. The enemy is taking cover, that gives them a -6 to their delta t, but the use of the ACOG sights gives the fire team a -4 bonus to delta. The GM and players roll dice. They need to beat a delta t of 4 by rolling less than or equal to 4 (10 base delta t - 6 cover modifier = 4). The GM rolls 18 for on of the enemies and one of the players roll 26, that's an 8 point difference, minus the ACOG modifier gives a 4 (8 - 4 = 4). Just enough to hit the enemy under cover. Damage is rolled as before and the enemy takes a serious wound. The two other players fire at the enemy, one misses and the other hits, once again injuring the enemy (rolls omitted for clarity).

With one group seriously injured and bleeding out and the other pinned down by the constant hammering of the SAW and assault riffles the enemy begins a retreat. They pull back between the trees and run away. Fire team A moves up to pursue. As they reach the top the see the enemy running through a clearing in the forest that leads to a local village. Assessing the distance at some 450 meters the team leader calls in fire support.

As FO  he calls out the following command to bring down two 60 mm rounds on a span of 50 m


The player rolls for the rounds

First round

Player roll 7 + 10 = 17
GM Roll 18 +14 = 32

Delta is 15, way to high to score a damaging hit. The second round is rolled for.

Player roll 1 + 18 = 19
GM Roll 11 + 7 = 18

Near direct hit with a delta of 1. The GM puts the enemies some 6 to 8 meters apart, four of them take a direct hit and the remaining 2 are over 10 meters from the blast center. Their modified delta is 3, still an deadly blast for them too. Damage is rolled for the blast using 2d10, the blast can't be stopped by the armor (concussion and frag damage rolls omitted). All enemy forces are neutralized.

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