THE GAMMA DAMAGE MANIFESTO

A complete and accurate (hopefully) overview of the ballistic mechanics

📋 TL;DR: The Basics

AP is king vs. armored stalkers. If your bullet can't punch through armor, it pretty much deals no damage. AP ammo is required against anything with serious protection.
HP ammo for mutants. Mutants skip the armor system entirely; damage is calculated directly, and HP rounds boost that raw damage significantly.
Keep your barrel above 70%. Below that both damage and AP drop hard. At 50% barrel you're dealing 28% less damage with 28% less AP.
Suppressors are a hidden buff. Both attached and integrated silencers give +7% to damage AND AP against NPCs.

1. Weapon Condition & Mechanics

Barrel condition influences both your damage and AP (AP gate formula and damage formula). It is critical to keep your barrel above 70%.

📉 Barrel Condition Penalties

Both damage and AP rely on your weapon's barrel condition

70%+: No penalty Degraded Severe Less than 50% dmg Unusable

🔢 What Weapon Stats & Upgrades Mean

STAT CARD = What the weapon description shows UPGRADE = What the upgrade at the technician does

Accuracy

STAT CARD Aim cone when Aiming Down Sights (ADS). Higher the better, 100% being perfect accuracy.

UPGRADE Improves the dispersion cone, both ADS and hip-firing.

Handling

STAT CARD Means nothing.

UPGRADE Improves hip-fire accuracy.

Recoil

STAT CARD Arbitrary formula result, doesn't mean much.

UPGRADE Decreases the camera climb when shooting, but not the camera shake.

Fire Rate

STAT CARD Weapon RPM.

UPGRADE Flat RPM increase (60% upgrade means generally +60 RPM).

Reliability

STAT CARD Chance for weapon parts to take damage when firing.

UPGRADE Reduces how fast weapon parts break down.

STAT CARD No upgrade paths for these

Damage: For reference only, not your "true" dmg as it gets drowned in a lot of other variables.
Max range: Distance at which the bullet despawns.
Muzzle velocity: Weapon bullet speed.

UPGRADE Does not appear in weapons cards

Flatness: Increases bullet speed and accuracy, but does not influence damage drop-off.

🔧 How Reliability works

Every shot you take triggers a "potential damage roll" for a random weapon part
For example, a weapon in perfect condition has a ~0.3% chance per shot to damage one of its parts
This increases dramatically the more damaged your weapon is
The part that breaks is chosen at random and will lose 5–12% of its current condition
Better reliability reduces the overall risk of parts taking damage

2. Special Mechanics

🔇 Silencer Bonus

Hidden; not shown anywhere in UI

+7% to both damage AND AP against NPCs
Applies to both attached and integrated suppressors

🎯 Sniper Class Bonus

Applied to most sniper rifles

Flat +5 AP bonus on top of the ammo's value
Applies to most snipers in game

⚡ Surrendered / Downed NPCs

Finishing mechanics

When an NPC is surrendering or wounded, any bullet deals double damage
Ensures downed enemies don't frustratingly tank finishing shots

Difficulty Multipliers

Both raw damage and AP are scaled by game difficulty on every hit.

Novice

×1.30

+25% damage & AP on every hit

Stalker

×1.05

Baseline

Veteran

×0.90

−15% damage & AP on every hit

On Veteran, every bullet deals ~15% less damage and has ~15% less AP than on the default Stalker difficulty.

3. Faction Resistances

🛡️ Player Armor: BR Class & BR%

BR Class: AP resistance ceiling. Bullets with higher AP than your BR Class will penetrate your armor, results are painful.
BR%: Flat damage reduction. Always applies, even on penetrating hits, but on a non-pen hit it stacks with BR Class.
Even when a bullet punches through, BR% still softens the hit. You will bleed on penetration (phrasing :o).

Some factions have extra resistances on top of their regular armor. Here's the breakdown:

Faction	Damage Resist	AP Resist	Practical Effect
Most Stalkers / Loners	×1.00	×1.00	Baseline, no resistance
Bandits	×1.00	×1.10	Slightly easier to penetrate than baseline
Monolith	×0.80	×0.90	20% damage resist + 10% AP reduction
UNISG (ISG)	×0.70	×0.65	30% damage + 35% AP reduction.
Sin	×0.30	×0.90	70% damage reduction + 10% AP reduction.
Zombied	×0.30	×1.00	70% damage reduction, same as Sin

The SIN urban legend

There's this long-standing myth that SIN don't have any armor, just a ton of health, so using HP ammo against them is the best.

They actually behave like every other NPC. The 70% resistance applies at the end of the damage calculation, but the regular ballistic mechanics are not affected.

See them as regular NPCs that have a 3 times bigger health bar (3.3 times exactly). AP ammo will still be much more efficient against armored SIN, FMJ against light/medium, and HP ammo against the "naked" ones (all the trenchcoat guys).

4. Hit Location Multipliers (vs. Stalkers)

Every hit location has its own damage multiplier, and the gap between a headshot and a foot shot is enormous.

👁️ Head (eye level)×3.65

💀 Jaw×3.00

🔴 Neck×2.70

⬛ Default (unmapped bone)×0.75

🫀 Pelvis / Spine×0.90

💪 Upper arm / Clavicle×0.70

🦴 Forearm×0.60

🦵 Thigh×0.55

🦿 Calf×0.45

✋ Hand×0.40

👟 Foot×0.30

Head = 12× the damage of a foot shot, 4× the damage of a torso shot.

5. Ammo Types & When to Use Them

🔴 AP: Armor Piercing

Required vs. heavily armored NPCs

The only effective option against exoskeletons
High penetration value; can fully punch through heavy armor
Expensive to buy and craft

⬛ FMJ: Full Metal Jacket

Cheap all-rounder

Default for lightly to moderately armored enemies
Good for breaking down armor before switching to AP
Flexible all-rounder

🩸 HP: Hollow Point

Best against mutants

High damage multiplier; hits much harder against unarmored targets
Near-useless against armor
When HP fails to penetrate, it suffers an extra damage penalty

6. Distance Drop-off & Air Resistance

This section is about the damage drop-off with distance (not gravity pulling your bullets down). It is the earliest modifier applied in the damage pipeline, before the AP gate check and the damage formula. It influences your ability to penetrate armor and your raw damage at exactly the same rate.

The drop-off penalty is a fixed ammo property. No weapon upgrade influences it.
The Flatness upgrade you can do increases bullet speed and accuracy, it does not reduce this damage drop-off at all.

Damage & AP retention (%) by distance

Distance	air_res 0.25 5.45x39 · 7.62x54 · 7.62x51	air_res 0.30 5.7x28 · 7.62x39	air_res 0.63 9x39 SP-5 · SP-6	air_res 0.80 9x18 · .357 · 9x21 SP10
25 m	98.2%	97.7%	92.3%	85.7%
50 m	96.5%	95.5%	85.6%	75.0%
100 m	93.2%	91.4%	74.9%	60.0%
200 m	87.2%	84.2%	59.9%	42.9%

AIR_RES is basically how much friction a bullet has. The higher, the bigger the damage drop-off.

A 9x18 pistol round at 200m hits with less than 43% of its base damage and 43% of its base AP. Enough for example to not penetrate an armor it would punch through at close range, and do effectively 0 damage. Or simply do 57% less damage against an unarmored enemy or mutant.

Fun facts:

5.56 retains slightly more damage than 5.45 (−5.3% vs −6.8% at 100m). Small difference, but technically NATO rifles have better range.
Small "good" calibers vary a lot: 5.7x28mm of the P90/MP7 will perform almost as good as Assault-rifle ammo like 5.45, while the 9x21mm SP10 of the Veresk has the same drop-off as an 9x18mm Makarov, the worst caliber in the game.

See the full Ammo Air Resistance Breakdown

air_res	Dmg loss at 100m	Ammo sections
0.01	−0.2%	`ammo_gauss`
0.05	−1.2%	`ammo_338_federal`
0.10	−2.4%	`ammo_pkm_100`
0.15	−3.8%	`ammo_23_eco`
0.20	−5.3%	`ammo_5.56x45_ap` · `ammo_5.56x45_fmj` · `ammo_5.56x45_ss190`
0.25	−6.8%	`ammo_5.45x39_ap` · `ammo_5.45x39_ep` · `ammo_5.45x39_fmj` · `ammo_7.62x51_ap` · `ammo_7.62x51_fmj` · `ammo_7.62x54_7h1` · `ammo_7.62x54_7h14` · `ammo_7.62x54_ap` · `ammo_7.92x33_ap` · `ammo_7.92x33_fmj`
0.30	−8.6%	`ammo_5.7x28_ss190` · `ammo_5.7x28_ss195` · `ammo_7.62x39_ap` · `ammo_7.62x39_fmj`
0.35	−10.4%	`ammo_7.62x25_p` · `ammo_7.62x25_ps` · `ammo_magnum_300` · `ammo_50_bmg`
0.63	−25.1%	`ammo_9x39_ap` · `ammo_9x39_pab9`
0.75	−34.9%	`ammo_9x19_ap` · `ammo_9x19_fmj` · `ammo_9x19_pbp` · `ammo_12.7x55_ap` · `ammo_12.7x55_fmj`
0.80	−40.0%	`ammo_357_hp_mag` · `ammo_4.6x30_fmj` · `ammo_9x18_ap` · `ammo_9x18_fmj` · `ammo_9x18_pmm` · `ammo_9x21_sp10` · `ammo_12x76_dart` · `ammo_12x76_zhekan`
0.82	−42.3%	`ammo_11.43x23_fmj` · `ammo_11.43x23_hydro`
0.85	−45.9%	`ammo_23x75_barrikada`
0.90	−52.9%	`ammo_23x75_shrapnel`
1.00	−71.4%	`ammo_12x70_buck` · `ammo_12x76_bull` · `ammo_20x70_buck`

7. The AP Gate: Formula Breakdown

Before any damage is applied, your bullet must pass a single check: can it punch through the target's armor? If it can't, damage drops to almost nothing; no other factor can compensate. This check happens first, every shot.

⚙ AP Value Assembly (hover a factor)

= ammo_kap × 10 Ammo's base AP value

+ leg shots: small AP bonus (less for buckshot)

+ head shots: small AP bonus (less for buckshot)

+ sniper-class weapon: flat AP bonus

× bone_ap_scale Armor density

× barrel_cond Barrel wear

÷ distance_falloff AP drops with range

× isg_res Faction AP resistance

× silencer_boost +7% with any suppressor

× difficulty_mult ×0.80–×1.30 by mode

× 0.80 global post-calculation adjustment

←

hover a factor to inspect it

Once the AP value is finalized, it's compared against the target bone armor value (the bone is the body part you hit) to determine which of three outcomes occurs:

✅ Full Penetration

AP equals or beats the armor

Full damage. The bullet passed straight through (no armor breaking).

⚠️ Partial Penetration

Some AP, but not enough

Reduced damage. You chip down the armor with every shot. Enough shots will break it to do full damage after.

🚫 No Penetration

Low / No AP

You do a tiny "concussion" amount of dmg. Armor still chips away, but at a very reduced pace.

8. The Damage Formula

If the AP check passes, this is what determines how much damage actually lands, ordered from most to least impactful.

⚙ Raw Damage: most to least impact

= hit_power Weapon base damage

× bone_dmg_mult Where you hit · ×0.30–×3.65

× k_hit Ammo type

× barrel_cond Wear and tear

÷ distance_falloff Distance drop off

× silencer_boost +7% with any suppressor

× sin_res Faction resistances

× bone_ap_scale Armor tier

× difficulty_mult ×0.80–×1.30

× ammo_mult ×1.0 for everything

× 1.1 Constant

←

hover a factor to inspect it

Weapon stat

Ammo property

Hit location

Environment

Faction modifier

Global constant

Hidden mechanic

9. Damage vs. Mutants (Separate System)

Mutants damage formula is much simpler than stalkers and completely skips the armor system. HP ammo is king vs. Mutants.

⚡ Melee Bonus · Knives and axes only · Melee weapons get +50% damage against mutants

Mutant	Base Multiplier	Critical Zone	Crit Multiplier
Snork	×1.10	Legs	×2.00
Boar	×0.95	None	No crit zone
Flesh	×0.95	Spine	×1.30
Bloodsucker	×0.85	Neck	×1.40
Psysucker	×0.85	Neck	×1.60
Lurker	×0.85	Head area	×1.50
Gigant (Pseudogiant)	×0.85	Core	×2.00
Chimera	×0.75	Head area	×2.00
Zombie (mutant)	×0.80	Head	×2.00
Fracture	×0.60	Spine	×2.20

10. Damage Cheat Sheet

❌ Wrong Ammo Type

HP and buckshot deal near-zero damage against armored enemies (until armor is broken down)
AP is required for anything with serious protection
FMJ works for light-medium armor and armor-breaking if no other option

❌ Faction Resistance

Sin / Zombied: 70% flat damage reduction
UNISG: 35% AP reduction + 30% damage reduction.
Monolith: 20% damage, 10% AP reduction.

❌ Barrel Below 70%

A 50% barrel does 28% less damage AND has a 28% AP penalty
Highest-impact on something that can be easily maintained