14  Appendix C: Loot System Internals

This appendix explains how BL4’s loot system decides what to drop, how rare each drop is, and how the bl4 toolkit estimates item rarity from serial data.

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14.1 Two Ways to Get a Legendary

Every legendary in BL4 reaches the player through one of two paths: dedicated drops or world drops. They use different probability models, and understanding the difference matters for estimating how rare a given item is.

14.1.1 Dedicated Drops

Dedicated drops are tied to specific bosses. Each boss has an ItemPoolList in NCS that names 1–3 legendary items, each assigned to a tier that determines its drop chance per kill:

Tier	Chance per kill
Primary	6%
Secondary	4.5%
Tertiary	3%

The first item in a boss’s pool is Primary (highest chance), the second is Secondary, and so on. When you kill a boss, the game rolls independently for each dedicated item — you can get zero, one, or (rarely) multiple legendaries from a single kill.

NoteShiny and TrueBoss Variants

Bosses have additional tier variants for special modes. TrueBoss (Chaos mode) raises the dedicated drop rate to 25%, making legendaries roughly 4x more common per kill. Shiny items are cosmetic variants at 1%, and TrueBossShiny at 3%.

This is the simplest path to a legendary: kill a boss, roll against a known percentage. If the Hellwalker is the Saddleback’s Primary drop, you have a 6% chance per kill. Farm the boss enough and you’ll get one.

14.1.2 World Drops

World drops are the other path. When any enemy dies, it can drop gear from the general loot pool. The rarity of that gear is selected by a weighted roll:

Rarity	Weight	Probability
Common	100.0	~94.18%
Uncommon	6.0	~5.65%
Rare	0.14	~0.132%
Epic	0.045	~0.0424%
Legendary	0.0003	~0.000283%

These weights come from the rarity_balance table in gbx_ue_data_table0.bin. The game sums all weights (106.1853) and picks a tier proportionally. Legendary is 0.0003 out of 106.1853 — roughly 1 in 353,000 world drops.

But that’s the chance of getting any legendary. The chance of getting a specific one is much lower. If you rolled Legendary on a world drop and the game selects from the Pistol pool, there are 9 legendary pistols across all manufacturers. Your chance of the specific one you wanted is the tier probability divided by the pool size: roughly 1 in 3.2 million.

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14.2 How a Drop Resolves

When an enemy dies, the game runs through several systems in sequence:

1. Base loot roll (Struct_EnemyDrops): determines what categories drop — guns, shields, grenades, class mods, currency. Each category has its own probability and quantity. A standard boss might drop 2 guns and 1 shield.

2. Rarity selection: for each item that drops, the rarity weight table determines what tier it rolls. This is where Common (94%) vs Legendary (0.0003%) is decided.

3. Pool selection: the game picks a specific item from the pool matching that rarity and item category. A legendary gun roll selects from itempool_guns_05_legendary.

4. Dedicated drop check: independently of the base loot, the boss’s dedicated pool is checked. Each assigned legendary rolls independently at its tier percentage (Primary 6%, Secondary 4.5%, etc.).

5. Luck modifier: the player’s luck stat modifies rarity weights at step 2. Higher luck increases the weight of rarer tiers. The exact formula uses GrowthExponent, BaseWeight, and GameStageVariance properties on the RarityWeightData class, but the specific curve is resolved at runtime and hasn’t been fully extracted.

Steps 2–3 and step 4 are independent paths. An item can appear as both a dedicated drop (guaranteed from a specific boss) and a world drop (from the general pool). The dedicated path is far more likely for any given item.

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14.3 Pool Sizes and Per-Item Odds

The rarity tier probability tells you how likely any legendary is. The pool size tells you how likely a specific one is. These are different questions.

Legendary items are organized into pools by manufacturer and weapon type. Jakobs Shotguns have 2 legendaries. Vladof Assault Rifles have 3. The bl4 toolkit’s drop_pools.tsv manifest captures these counts.

For world drops, the game selects across all manufacturers within a weapon type. If it rolls “legendary pistol”, it’s choosing from all 9 legendary pistols (Jakobs, Daedalus, Tediore, etc. combined). The per-item probability for a specific legendary pistol from a world drop is:

tier_probability / world_pool_size = 0.000283% / 9 = ~0.0000314%

That’s roughly 1 in 3.2 million world drops for a specific legendary pistol. This is why dedicated boss farming (6% per kill for a specific item) is orders of magnitude more efficient than hoping for world drops.

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14.4 Rarity Estimation

The bl4 toolkit can estimate an item’s rarity from its serial string. The rarity_estimate() method on ItemSerial combines several data sources:

1. Rarity tier: decoded from the serial’s inv_comp part (comp_01 through comp_05)
2. Tier probability: looked up from the rarity weight table
3. Manufacturer and type codes: extracted from the serial’s token stream (VarInt-first for weapons, VarBit-first for equipment)
4. Pool data: matched against the compile-time drop_pools.tsv manifest for legendary count, world pool size, and boss source count

For legendaries, the estimate divides the tier probability by the world pool size to get per-item odds. For other rarities, it reports only the tier probability (since the pool selection is less meaningful — a “common Jakobs pistol” isn’t a specific item the way a legendary is).

Rarity estimate:
  Tier: Legendary (0.000283%, ~1 in 353,490)
  Pool: Jakobs Shotgun (2 legendaries, 9 in world pool)
  Per-item: ~1 in 3,181,413
  Boss sources: 2

Warning

These are estimates, not exact values. The rarity weight table is extracted from NCS data tables and matches community testing results, but luck modifiers, game stage scaling, and category selection probabilities are resolved at runtime and aren’t captured here.

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14.5 What We Know vs. Don’t Know

Extracted from NCS data:

• Dedicated drop probabilities per tier (from Table_DedicatedDropProbability)
• Rarity weights for the base tier selection (from rarity_balance)
• Boss-to-legendary assignments and tier ordering (from itempoollist.bin)
• Pool sizes per manufacturer/weapon type (from drops manifest)
• Enemy drop category probabilities and quantities (from Table_EnemyDrops)

Not yet extracted:

• Luck system curve (how much luck shifts rarity weights, resolved at runtime)
• Category selection probabilities within a rarity tier (shield vs weapon vs grenade)
• Game stage scaling effects on weights (the GrowthExponent / GameStageVariance interaction)
• Tier assignments for most dedicated drops (only 1 of 133 boss drops has explicit tier context in NCS — the rest are inferred by pool ordering)

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14.6 NCS Data Sources

Drop information is stored across several NCS files within pak archives:

File	Contents
itempoollist.bin	Boss → legendary item mappings with tier assignments
itempool.bin	Item pool definitions, rarity weights, world drop membership
gbx_ue_data_table0.bin	Table_DedicatedDropProbability, rarity_balance, Table_EnemyDrops, Table_LootableBalance
loot_config.bin	Global loot configuration parameters
preferredparts.bin	Part preferences for item generation

Numeric values in these tables are stored as strings in NCS ("0.060000", "1.500000"). The binary section’s bit-packed indices point into the string table where these values live.

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14.7 Reference: Dedicated Drop Probability Tiers

From Table_DedicatedDropProbability in gbx_ue_data_table0.bin. Schema defined in Struct_DedicatedDropProbability.uasset with a single DoubleProperty field.

Tier	Row Name	Index	Probability
Primary	Primary_2_<GUID>	2	6%
Secondary	Secondary_4_<GUID>	4	4.5%
Tertiary	Tertiary_6_<GUID>	6	3%
Shiny	Shiny_9_<GUID>	9	1%
TrueBoss	TrueBoss_12_<GUID>	12	25%
TrueBossShiny	TrueBossShiny_14_<GUID>	14	3%
Quaternary	Quaternary_16_<GUID>	16	0%

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14.8 Reference: Rarity Weights

From rarity_balance in gbx_ue_data_table0.bin. Total weight: 106.1853.

Tier	Component ID	Weight	Probability
Common	comp_01_common	100.0	94.18%
Uncommon	comp_02_uncommon	6.0	5.65%
Rare	comp_03_rare	0.14	0.132%
Epic	comp_04_epic	0.045	0.0424%
Legendary	comp_05_legendary	0.0003	0.000283%

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14.9 Reference: Enemy Drop Fields

From Struct_EnemyDrops. Each enemy tier has rows in Table_EnemyDrops controlling what categories drop and in what quantities.

Field	Description
Guns_Probability	Chance to drop guns
Guns_HowMany	Number of guns
Shields_Probability	Shield drop chance
Shields_HowMany	Number of shields
GrenadesOrGadgets_Probability	Grenade/gadget chance
GrenadesGadgets_HowMany	Number to drop
ClassMods_Probability	Class mod chance
ClassMods_HowMany	Number of class mods
RepKits_Probability	Repair kit chance
RepKits_HowMany	Number of repair kits
Enhancements_Probability	Enhancement chance
Enhancements_HowMany	Number of enhancements
CurrencyOrAmmo_Probability	Currency/ammo chance
CurrencyAmmo_HowMany	Amount
EXP_Multiplier	Experience multiplier
Rarity_Modifier	Rarity boost

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14.10 Reference: Boss → Legendary Mappings

Extracted from itempoollist.bin. Each boss has 1–3 dedicated legendary drops. The first item listed is Primary tier (6%), second is Secondary (4.5%), third is Tertiary (3%).

Boss	Primary Drop	Secondary Drop	Third Drop
Arjay	ORD_SR Fisheye	DAD_SG HeartGun	-
Backhive	VLA_SR StopGap	-	-
Bango	JAK_PS Phantom_Flame	BOR_SM Prince	-
BatMatriarch	TOR_SG Linebacker	BOR_SM hellfire	-
BattleWagon	VLA_SR Finnty	TOR_AR Bugbear	-
BlasterBrute	JAK_SG Slugger	MAL_SG Kaleidosplode	-
Bloomreaper	MAL_SG Mantra	-	-
CityCat	DAD_SG Bod	-	-
CloningLeader	TED_PS Sideshow	-	-
Destroyer	JAK_SR Boomslang	BOR_SG Convergence	-
Donk	JAK_AR Rowdy	TED_PS Inscriber	-
Drillerhole	ORD_AR GMR	MAL_SR Katagawa	-
DroneKeeper	ORD_PS Bully	TED_AR DividedFocus	-
FirstCorrupt	JAK_PS KingsGambit	DAD_PS Rangefinder	-
GlidePackPsycho	TOR_SG LeadBalloon	-	-
Grasslands_Commander	BOR_SG GoldenGod	BOR_SG GoreMaster	VLA_SM Onslaught
Grasslands_Guardian	TED_SG HeavyTurret	-	-
Hovercart	VLA_AR WomboCombo	TOR_AR PotatoThrower	-
KOTOMotherbaseBrute	TED_PS ATLien	-	-
KotoLieutenant	ORD_PS RocketReload	VLA_AR DualDamage	-
LeaderHologram	TED_AR Chuck	-	-
MeatPlantGunship	MAL_SR Asher	-	-
MeatheadRider	VLA_AR Lucian	-	-
MeatheadRider_Jockey	JAK_SG Hellwalker	-	-
MountainCommander	TED_PS RubysGrasp	-	-
MountainGuardian	VLA_SM KaoSon	-	-
Pango	BOR_SR Stray	-	-
Redguard	VLA_AR WF	JAK_AR Rowan	JAK_AR BonnieClyde
RockAndRoll	JAK_PS QuickDraw	JAK_SG TKsWave	-
ShatterlandsCommanderElpis	MAL_SM OhmIGot	-	-
ShatterlandsCommanderFortress	TOR_PS QueensRest	-	-
ShatterlandsGuardian	TED_SG Anarchy	-	-
SideCity_Psycho	ORD_AR Goalkeeper	JAK_PS SeventhSense	-
SkullOrchid	TOR_PS Roach	-	-
SoldierAncient	DAD_AR Om	-	-
SpiderJumbo	ORD_PS NoisyCricket	-	-
StealthPredator	BOR_SR Vamoose	-	-
StrikerSplitter	DAD_SM Luty	-	-
SurpriseAttack	DAD_SM Bloodstarved	-	-
Thresher_BioArmoredBig	JAK_SR Truck	-	-
Timekeeper_Guardian	MAL_SM PlasmaCoil	DAD_AR StarHelix	-
Timekeeper_TKBoss	ORD_SR Symmetry	JAK_SR Ballista	-
TrashThresher	MAL_SG Kickballer	VLA_SM BeeGun	-
UpgradedElectiMole	DAD_PS Zipgun	JAK_SG RainbowVomit	-

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14.11 Reference: Item Pools

14.11.1 Boss Pool Lists

Pool	Description
ItemPoolList_Enemy_BaseLoot_Boss	Standard boss drops
ItemPoolList_Enemy_BaseLoot_BossRaid	Raid boss drops
ItemPoolList_Enemy_BaseLoot_BossMini	Mini-boss drops
ItemPoolList_Enemy_BaseLoot_BossVault	Vault boss drops
ItemPoolList_*_TrueBoss	True Boss (Chaos mode) variants

14.11.2 Rarity-Tiered Weapon Pools

Pool	Rarity
itempool_guns_01_common	Common
itempool_guns_02_uncommon	Uncommon
itempool_guns_03_rare	Rare
itempool_guns_04_epic	Epic
itempool_guns_05_legendary	Legendary

14.11.3 Special Pools

Pool	Description
ItemPool_FishCollector_Reward_Legendary	Fish collector reward
ItemPool_BlackMarket_Comp_BOR_HW_DiscJockey	Black Market exclusive
ItemPool_BlackMarket_Comp_BOR_HW_Streamer	Black Market exclusive

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14.12 Reference: Drop Source Types

Type	Description
Boss	Dedicated boss drop with per-kill tier probability
Mission	Side/main mission reward (guaranteed on completion)
BlackMarket	Black Market vendor exclusive
Special	Fish Collector, challenges, event rewards
WorldDrop	General legendary pool (rarity-weighted)

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14.13 Reference: Item Composition in NCS

Legendary items follow a consistent naming pattern:

MANUFACTURER_TYPE.comp_05_legendary_NAME

Examples: JAK_SG.comp_05_legendary_Hellwalker, MAL_SM.comp_05_legendary_PlasmaCoil, DAD_SHIELD.comp_05_legendary_angel.

Component	Rarity
comp_01_common	Common
comp_02_uncommon	Uncommon
comp_03_rare	Rare
comp_04_epic	Epic
comp_05_legendary	Legendary

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14.14 Reference: Codes

14.14.1 Weapon Types

Code	Type
AR	Assault Rifle
PS	Pistol
SG	Shotgun
SM	SMG
SR	Sniper Rifle
HW	Heavy Weapon

14.14.2 Manufacturers

Code	Manufacturer	Specialty
BOR	Ripper	-
DAD	Daedalus	Shields
JAK	Jakobs	High damage, slow fire
MAL	Maliwan	Elemental
ORD	Order	-
TED	Tediore	Throw-to-reload
TOR	Torgue	Explosives
VLA	Vladof	High fire rate

14.14.3 Gear Slots

Slot	Type Code
Weapon 1–4	AR, PS, SG, SM, SR, HW
Shield	SHIELD
Grenade	GRENADE
Class Mod	CM
Artifact	ARTIFACT
Repair Kit	RK, REPAIR_KIT

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14.15 Reference: Drops Manifest Format

The drops.json manifest generated by bl4 drops generate contains:

{
  "version": 1,
  "probabilities": {
    "Primary": 0.06,
    "Secondary": 0.045,
    "Tertiary": 0.03,
    "Shiny": 0.01,
    "TrueBoss": 0.25,
    "TrueBossShiny": 0.03
  },
  "drops": [
    {
      "source": "MeatheadRider_Jockey",
      "source_display": "Saddleback",
      "source_type": "Boss",
      "manufacturer": "JAK",
      "gear_type": "SG",
      "item_name": "Hellwalker",
      "item_id": "JAK_SG.comp_05_legendary_Hellwalker",
      "pool": "itempool_jak_sg_05_legendary_Hellwalker_shiny",
      "drop_tier": "Primary",
      "drop_chance": 0.06
    }
  ]
}

The companion drop_pools.tsv summarizes legendary counts and boss source counts per manufacturer/weapon type pool. It is embedded at compile time for the rarity estimation API.

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14.16 Reference: Loot System Classes

Classes relevant to the loot pipeline, discovered via memory analysis:

14.16.1 Pool System

Class	Role
ItemPoolDef	Defines a loot pool (/Script/GbxGame.ItemPoolDef)
ItemPoolEntry	Single item in a pool
ItemPoolListDef	Ordered list of pools (boss drop lists)
ItemPoolSelectorDef	Selection logic between pools

enum class ELootPoolTypes {
    All = 0,
    BaseLoot = 1,
    AdditionalLoot = 2,
    DedicatedDrops = 3,
    GearDrivenDrops = 4,
    MAX = 5
};

14.16.2 Rarity Resolution

Class	Role
RarityWeightData	Weight configuration (BaseWeight, GrowthExponent, GameStageVariance)
LocalRarityModifierData	Local rarity modifiers (area/event bonuses)
InventoryRarityDataTableValueResolver	Resolves rarity values from DataTables at runtime
InventoryRarityDef	Rarity tier definition
LootChanceDefinedValueRow	DataTable row for loot chance configuration

14.16.3 Luck System

Class	Role
LuckGlobals	Global luck settings
LuckCategoryDef	Luck category definition
LuckCategoryAttribute	Per-category luck attribute
LuckCategoryValueResolver	Resolves luck values at runtime

Three luck category groups: LuckCategories (base), EnemyBasedLuckCategories (per-enemy), PlayerBasedLuckCategories (player-specific).

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Data from NCS file extraction and live memory analysis.