Fallout 4 legendary weapons

In Fallout 4, Legendary weapon effects are special modifications applied to basic weapons that use special prefix modifiers, not obtainable elsewhere. Legendary weapons cannot be scrapped, but can be further modified and traded with vendors. An on-screen notification informs the player that they have picked up a Legendary weapon; in the Pip-Boy's inventory screen, they are marked with a star. Unlike unique weapons, they are randomized and are never named. Legendary effects can provide the same effects as most unique weapons, though; a "Relentless .44 pistol" is identical to Kellogg's pistol. Like unique weapons, Legendary weapons can be given a custom name.

Location
Legendary weapons are most commonly found on the bodies of "Legendary" variants of enemies. These enemies spawn at random locations throughout the Commonwealth, the Island, and Nuka-World, depending on the difficulty setting, and will almost always have a legendary piece of equipment in their inventory, either a Legendary weapon or Legendary armor. Legendary weapons can also be awarded for completing certain quests and can be found in tribute chests, albeit rarely.

Legendary weapon prefixes
Most legendary weapons will have one of the following prefixes, which acts as a short description of the weapon's effects.

Unique weapon effects
Some effects are not randomly generated legendary weapons. These are known as unique weapon effects, and they can only be found by adding the effect through commands, or getting the unique weapon that they are found on.

Detailed explanation
The legendary weapons and armor that are found on enemies are random; there are certain "pools" of said weapon and armor types specifically tied to the type of enemy and location in which they are spawned. Certain legendary variants of weapons encountered early in the game - such as laser muskets and pipe pistols - will not spawn on Gunners, but will always have a chance to be found on legendary enemies with an area-specific low level cap (e.g. the ghouls in Super Duper Mart).

Berserker weapon effects

 * The Berserker effect works in reverse as well; having an excessive armor rating will actually decrease the weapon's damage to effectively nothing. The effect is not decreased by radiation resistance.
 * The "Berserker" mod occasionally causes a miscalculation of damage; the damage decreases if not wearing any base clothing (army fatigues, vault jumpsuit, jeans and t-shirt, etc.).
 * The damage calculation is also not shown until the weapon is equipped, and then it will also translate the effect onto other weapons when viewed in the inventory.
 * This weapon mod can be added to ranged weapons via console, e.g. "Berserker's Shielded Gauss rifle"

Crippling weapon effects

 * The Crippling effect can sound misleading; it does not increase a weapon's attack power, but its limb damage. Therefore, enemies will still take the same amount of hits to defeat, but their limbs will be easier to cripple.

Damage Reduction armor & weapon effects
All Damage Reduction (i.e. Hunter's, Ghoul Slayer's, Assassin's, Exterminator's, Troubleshooter's, Mutant Slayer's, Cavalier's, and Sentinel's) mod stacks with the armor and weapon mod of the same name.
 * Equipping 7 pieces (weapon, helmet, left arm, left leg, right arm, right leg, chest) will negate all damage; except fire or poison damage. This causes enemies to panic and run away, due to how Fallout 4's AI is programmed.

Defiant weapon effects

 * The Defiant effect permanently doubles the damage for any weapon with a capacity of 1, as they are technically always on the "last round" before reloading.
 * However, explosive weapons such as the Fat Man see almost no benefit, as the damage multiplier does not apply to explosive damage.

Elemental weapon effects
Freezing, Incendiary and Plasma infused mods are most effective on automatic or low base damage weaponry.
 * The bonus damage from the Freezing and Plasma infused prefixes scales with all relevant damage modifiers. That is, with 5 ranks in Gunslinger, a Plasma-Infused Pipe Pistol will deal 20 points of bonus energy damage. However, if you had 5 ranks in Commando but no ranks in Gunslinger, that pistol would only deal the base 10 points of energy damage. This bonus damage is also affected by Bloody Mess, Astoundingly Awesome Tales #07 and Lone Wanderer. For melee weapons, the damage is affected by the Rooted perk as well, but does not receive bonus damage from the Strength stat.
 * While only doing 10 points of damage each, the damage from the weapon effect is shown directly under the weapon's normal damage as additional damage, thus enabling it to be increased with respective perks. For example, at rank 4 of Big Guns and rank 3 of Lone Wanderer, a Freezing minigun will do 23 points of ballistic damage and 28 points of cryo damage, making it superior to other bonus damage weapon effects such as Wounding and Incendiary mainly because the cryo damage is instant and not a damage over time effect. The Freezing and Plasma infused mods are also vastly superior on all other low-damage, fast-attacking weapons such as rippers and automatic weapons. Other perks that directly or passively affect damage further increase the damage of these weapon effects as well.
 * Freezing weapons also have great synergy with critical focused builds due to their freezing effect on critical hits. With Critical Banker, Four Leaf Clover and a freezing weapon with a low Action Point cost, each instance of combat can be opened by freezing up to 5 combatants (4 without the Far Harbor add-on) -reducing a numbers advantage or allowing free attacks for a time. Use of Overdrive allows this tactic to be used without a V.A.T.S. focused build when paired with a rapid firing weapon.
 * Certain enemies, such as mirelurks, deathclaws and enemies wearing power armor cannot be frozen.
 * Plasma Infused weapons share many characteristics with "true" plasma weapons including the latters' high chance of spawning several units of nuclear material on any defeated enemy. Gooifying the target with a critical kill shot virtually guarantees this, thus providing an abundant source for this otherwise rare crafting component, especially for player characters specializing in critical hit chance.
 * Incendiary weapons' damage does not stack, making the damage fall far behind other legendary affixes. Considering Incendiary does 15 points of damage over time and Wounding does 25, Incendiary is a nearly useless effect by comparison and can be replaced by Wounding on all weapons with greater efficiency.
 * Poisoner's is generally considered a fairly terrible effect because of the fact that the majority of creatures are resistant (or immune) to poison, and it deals the damage over a very long period of time. While its damage effect totals out to be higher than other modifiers, it is also weaker due to its lack of immediate potency.

Explosive weapon effects
Explosive mod is exceptionally powerful when paired with weapons with low base damage that boast a high rate of fire or shoot several projectiles, as each bullet or pellet of scattered shot will produce an independent explosion, and the flat damage bonus itself is usually more than percentile bonuses.
 * The damage dealt by the Explosive prefix is increased by the Demolition Expert perk as well as the Explosives bobblehead. With 4 ranks in Demolition Expert plus the Explosives bobblehead, Explosive weapons actually deal 32.25 explosive splash damage per pellet.
 * The area-of-effect damage dealt by the Explosive prefix is also increased by Commando/Gunslinger/Heavy Gunner/Rifleman perks. Heavy Gunner increases damage more than its counterparts.
 * Minigun: the best example for the first case. Considering a base damage of 8 points per bullet, adding 15 points of explosive damage to every shot fired effectively triples the weapon's damage output even without taking the splash damage into account, as opposed of the "Mighty" Legendary Mod adding only 25% of 8 which is +2 to an end result of 10 versus 8 + 15 resulting in 23. Installing barrel mods somewhat weakens this effect, but it remains quite powerful regardless. The fact that the splash radius makes the minigun's inherent spread largely irrelevant only serves to further improve the effect's usefulness - as long as the weapon is aimed at the player character's target, it will hit it even if the bullets themselves don't connect.
 * For example, a base minigun does 8 damage per bullet. With explosive, it does 8+15=23 damage per bullet. With rank 1 heavy gunner, you get +20% damage, but that bonus is applied to both the original 8 and the explosive 15. It therefore does 1.2 * (8 + 15) = 27 (rounded down) per bullet, instead of just (1.2 * 8) + 15 = 24 damage. At rank 5, it therefore does 2 * (8 + 15) = 46 damage per round, and NOT just (2 * 8) + 15 = 31 damage.
 * The Demolition Expert perk only affects the explosive component. With rank 1, for example, the weapon then does 8 + (1.25 * 15) = 26 damage per bullet. Similarly, maxed out the perk does 8 + (2 * 15) = 38 damage per round
 * Taken together, this allows the minigun to far exceed its traditional base damage, doing a maximum of 2 * (8 + (2 * 15)) = 76 damage per round, instead of the 46 damage per round as with other weapon classes.
 * Shotguns: benefit from explosive shots, multiplying their damage output based on the amount of shot/beams in each round fired.
 * An explosive combat shotgun with an advanced receiver (firing 8 projectiles instead of a base of 7), 3 star Demolition Expert and 5 star Rifleman can do up to 2*100+(15*1.75)*8=410 Base Damage per shot. (V.A.T.S. does not take every projectile in to account, and the weapon information on your Pip-Boy will fall short of the total damage)
 * This effect can only spawn on ballistic weaponry. Laser, Explosive and other weapons cannot have this effect without mods or console commands. This is due to various glitches caused by other kinds of weapons.
 * Explosive weapons such as the Broadsider and missile launcher will not work with certain legendary effects, such as Incendiary and Explosive. These effects appear to replace the explosive damage with their own effect, reducing the weapon's damage dramatically.
 * Laser weapons, such as the Laser rifle appear to function as intended at first as the visual effect functions correctly. However the actual damage of the explosion becomes completely ludicrous, instantly killing almost any foe in a single hit. A scatter laser rifle with this effect can outdamage any other weapon in the game, turning Legendary Mythic Deathclaws into a pile of limbs in a single shot. They will also almost always kill the player if they get caught by any recoil damage. This could be caused by some form of damage calculation error with laser weaponry.
 * Other Energy don't function correctly at all, and results in the gun changing its firing animations and sound effects from a laser/plasma/fire-based weapon to a ballistic gun, including a stream of casings being ejected from somewhere behind or inside the weapon.
 * When firing Explosive-enhanced weapons in close quarters, their splash damage can injure the shooter just like grenades, missiles or any other explosive munitions do.
 * Killing almost any sort of enemy with a powerful Explosive-enhanced automatic weapon in V.A.T.S. will often launch the target dozens of meters into the air and hurl them a considerable distance, especially if one of the salvo's first hits already dealt the killing blow. This can make finding the corpse for looting difficult at times.
 * Explosive shots fired from a weapon with this modifier, and unlike shots fired with weapons bearing the Incendiary effect, will not ignite oil pools.

Furious weapon effects

 * The Furious effect increases a weapon's damage by 15% after each consecutive hit. That is, hitting an enemy three times in a row will increase the damage of the third hit with 30%. The counter of consecutive hits is reset after not hitting the target for 10 seconds, or after hitting another target. Simply missing a target does not affect the counter.
 * The Furious effect does not increase the radiation damage on the radium rifle, only ballistic. This is because the radiation damage is actually an enchantment that is not adjusted by damage modifiers.

Instigating weapon effects

 * The Instigating mod is at its best when found on powerful semi-automatic or bolt-action weapons, like all kinds of sniper rifles and especially the Gauss rifle. Considering that most, if not all, hostiles will be at full health when the player character is setting up a sniper ambush against an enemy stronghold, automatically dealing double damage against them seriously increases the chance of handing out a chain of one-hit-kills. Slap on a silencer before going to work and you can clear entire outposts without ever being detected. A fully charged sneak shot from a fully upgraded and silenced Instigating Gauss rifle, coupled with maxed Rifleman, Ninja and Mister Sandman perks, is a virtually guaranteed one-hit-kill against anything that roams the Commonwealth, regardless of distance, difficulty setting and whether it was a headshot or not.

Irradiated weapon effects

 * The Irradiated mod is potentially very powerful, scaling extremely well with higher difficulties and higher levels. When combined with the Nuclear Physicist perk, Irradiated weapons do 100 points of radiation damage (10% of max health). At lower levels and difficulties, the normal damage of the weapon may dwarf the 10% max health reduction (due to how radiation damage in Fallout 4 works). Indeed, even at high levels, properly upgraded and geared weapons may also do much better against many enemies. However, high-level human enemies may have over 1,000 health and, due to radiation damage ignoring player damage penalties in higher difficulties, a single irradiated shot could be worth hundreds of damage, highly effective on automatic weapons (even a low-level auto pipe pistol). Irradiated Gamma guns and Radium rifles are even more extreme.
 * The Irradiated mod is also very effective against legendary enemies, as it reduces the maximum health points of the target, preventing the healing effect of the mutation.

Kneecapper weapon effects

 * The Kneecapper mod's effect can sound misleading. It does not increase limb damage in any way; rather, each projectile/attack has a 20% chance to instantly cripple the target's legs.
 * This makes the Kneecapper effect most effective on automatic weapons or shotguns, as any single hit will trigger the effect regardless of damage.
 * The effect will trigger regardless of where the target is hit.
 * The most obvious targets are all types of melee-focused enemies from mole rats or feral ghouls all the way up to deathclaws. A short burst will usually be enough to turn them into sitting ducks, making them unable to close in on their target and therefore depriving them of almost any threat potential.
 * While enemies with ranged weapons still pose a threat even with their legs crippled, they collapse to the ground when the effect triggers and remain prone for some seconds, taking them out of the fight temporarily. If that enemy was shooting from behind cover, their line of sight will be broken, thus preventing them from attacking any longer until the Survivor digs them out.
 * Kneecapper weapons should not be used to attack flying Vertibirds. When the crippling effect triggers, it will cause a bug by instantly destroying the flyer's turbines while making it impossible to destroy completely. This can be especially aggravating during the Rockets' Red Glare quest for the Railroad where you are tasked with eliminating an attacking Brotherhood of Steel Vertibird.
 * Kneecapped Vertibirds will hover just above ground level for as long as you remain in the area. Walking away should trigger their crash into the ground, killing the Vertibird.

Never ending weapon effects

 * The Never ending mod sets ammo capacity equal to the amount one is carrying, eliminating the need to reload. However, this mod can make certain weapons unstable. For example, a laser musket with the effect will consume all ammunition for the gun that is currently in the player inventory. However, as the musket uses its "energy reservoir" to store the crank charge, and this mod sets the reservoir size to the amount of ammo in your inventory, the mod effectively removes the crank limit. Since the damage of the weapon is based on how many cranks you can give it, this gives that single shot a devastating amount of damage. For example, at 200 cranks, a single shot can do about 30,000 damage.
 * In addition, powerful weapons that are balanced only by reload times, such as the double-barrel shotgun, will have that balance thrown out the window, turning it into a semi-automatic version. Please note that it is not normally possible to find heavy weapons (like the Fat Man or missile launcher) with this effect.
 * The Gatling laser has a unique bug relating to this effect. When fired and the player has at least 3 Fusion cores, the weapon will seemingly drain most of one core, however it will then no longer drain any further energy, resulting in the weapon having infinite ammo. If the player has less than 3 cores, the weapon will instantly drain them. Oddly, despite its buggy nature, a unique Gatling laser has this effect: Aeternus.

Nimble weapon effects

 * The Nimble modifier can be used by players wearing power armor to significantly boost their movement speed while sneaking. Although it can be difficult to walk around with certain weapons scoped (such as weapons with actual scopes), aiming down the sights of a small nimble pistol while sneaking in power armor results in a faster movement speed faster than normal standing running in power armor.
 * The movement speed while overburdened is the same as that while not overburdened - thus, a player can continue moving at running speed while overburdened simply by aiming down a weapon's sights continuously.

Nocturnal weapon effects
The Nocturnal modifier works following way:
 * Morning, Afternoon, Evening: 33%
 * Sunrise, Sunset: 66%
 * Night: 150%
 * Midnight: 200%

Penetrating weapon effects

 * The Penetrating mod's effectiveness, due to the exponential curve of armor based damage reduction, does not vary with the target's armor after a certain threshold. When the target's armor rating is less than 15% of the weapon's damage, which is very little, the damage is applied in full (meaning 99%) and the Penetrating effect wasted. If the armor rating is between 15% and 21.4%, a Penetrating weapon will still do full (99%) damage while a standard variant will already experience damage reduction, making the Penetrating weapon increasingly better by comparison (0% to 13.9% more damage). Past this point, which is still very low for semi-automatic weapons, having the Penetrating mod means a static 13.9% increase in effective damage.
 * When looking at the effect in the Creation Kit, the effect used to determine the percentage of Armor/Energy penetration is the same as the Rifleman perk, which means that unlike what the Penetrating effect description says, this effect does not ignore the enemy Energy resistance.

Rapid weapon effects

 * The Rapid mod has an interesting effect when found on a Gatling laser. Due to the way the game calculates bonus percentages, a Gatling laser with the Charging Barrels mod (-75% rate of fire) and the Rapid affix (+25% rate of fire) shoots twice as fast as it normally would, at 50% of the standard firing rate instead of 25%, which is still almost as fast as an assault rifle. This effectively doubles the gun's "damage per minute" output with a mod installed that already quadruples its per-shot damage, creating a truly devastating weapon that is extremely cost-efficient in terms of ammo usage. This effect is essentially what makes the unique Final Judgment variant so deadly. It also largely eliminates the Charging Barrels' greatest drawback (very low rate of fire) and thus makes it far more useful in fights against numerous weak and/or fast-moving enemies that are normally hard to hit, in addition to still inflicting ludicrous amounts of damage to anything else as well, no matter how heavily armored it might be.

Resolute weapon effects

 * The slow time from the Resolute effect occurs not only when shooting the second to last bullet but also when player character switches to the weapon with Resolute effect and a single round in the magazine:
 * A Resolute laser musket's time slow effect will occur upon the first crank, the player character must wait a short while before resuming cranking or the effect will not activate.

Slow Time weapon effects

 * The slow time effects from Dead Eye and Resolute are not canceled when switching to a different weapon thus technically allowing the effect to be partially transferred to it. This can be useful to make a better aimed sniper shot.

Staggering weapon effects

 * The Staggering prefix, when applied to a weapon with a very high fire rate (such as a ripper or flamer) or with multiple projectiles (such as a combat shotgun), will usually render the target helpless while being hit, reducing the toughest enemies to easy targets. This is especially effective when combined with other effects that can cause stagger, such as Commando or the knockdown effect of Sniper.
 * Based on stats from the Creation Kit, non-automatic weapons have a 30% to stagger a target each hit, while automatic weapons have 5%. One-hand or two-hand melee weapons have 50%, which allows weapons like The Harvester to stunlock an opponent.

Two-shot weapon effects

 * The Two shot modifier makes the weapon fire an additional projectile while still using only 1 ammo and at slightly reduced accuracy and can greatly increase recoil. The game adds the base (i.e. unmodded) damage of the gun to its current (modded) damage and then splits the combined damage evenly into two projectiles. For example, an unmodded Hunting rifle does 37 damage, while a .50 cal Hunting rifle does 64. So a Two-shot .50 cal Hunting rifle does 64+37 = 101 damage, resulting in two projectiles hitting for 50.5 each. The damage value displayed in the Pip-Boy is the damage of both projectiles combined.
 * It is not normally possible to find heavy weapons like the missile launcher or harpoon gun with this effect.
 * The game uses the combined damage to calculate the damage reduction multiplier of the target's armor, so when both bullets hit, it does the same damage that one bullet with the combined damage would do. Therefore a Two-shot weapon will do more damage than the shots of an identically modded normal weapon and its unmodded variant combined.
 * As only the weapon's base damage is added, a shot from a heavily modded version of a two-shot weapon may do significantly less than twice the damage of a shot from a non-legendary version of the weapon modded in the same fashion. It does, however, help punching through the target's armor, delivering more of the applied mods' damage.
 * Two shot has an ability to provide a slight damage up when applying weapon mods which reduce damage. As it uses the base damage for the second projectile's damage, a mod which lowers damage (such as an automatic receiver) will be averaged with a higher damage rating, giving both bullets slightly higher damage than the weapon would have without Two shot.
 * Upgrading a Two Shot Fat Man like Big Boy with the MIRV modification will result in the weapon carpet-bombing a huge area with twelve mini nukes instead of six while simultaneously and massively reducing its range, so be aware of your surroundings and launch trajectory in order to avoid damaging yourself.
 * When applied to an unmodified double barrel shotgun, it will double its base damage, but cause it to have zero accuracy, meaning it will only be effective at point blank range.
 * A Two Shot railway rifle will shoot two railway spikes, and because of this, enemies killed with such a weapon have a chance to drop two railway spikes instead of just one.
 * All Two Shot weapons can deliver up to double the damage of their base, unmodified variant. This means that a Two Shot weapon can easily do more damage than an identical weapon with one of the race-specific damage prefixes (Assassin's, Exterminator's, Ghoul slayer's, Hunter's, Mutant slayer's, Troubleshooter's). For example, given two fully identically modified .50 sniper rifles, one a Ghoul Slayer's and one a Two Shot, the former will do 64 base damage to any enemy and (64 * 1.5) = 96 to Ghouls; while the latter will do a consistent (as demonstrated above) 101 damage to any and all enemies, Ghouls or not. This means that any Two Shot weapon effectively obsoletes all race-specific variants of most weapons. This can exclude some rapid fire weapons however, as the extra recoil may make them difficult to use.
 * Two Shot fully stacks with the laser musket 'cranking' mechanic and doubles or almost triples its output. As a four-crank capacitor and long barrels are available with Science! 1, a legendary drop with 248 damage per shot is potentially available very early.
 * While rare, a Two Shot weapon can miss on a critical hit.

VATS enhanced weapon effects

 * Ranged weapons will use 25% less AP and gain 33% increased hit chance; on melee weapons will have their AP cost by reduced by 40%.
 * Low AP-cost weapons such as the Deliverer can easily queue 16 attacks, the maximum amount per use of V.A.T.S.

Wounding weapon effects

 * Wounding is of particular note because there is no such thing as bleed resistance and bleed effects stack, and many enemies in the game are challenging to kill because their resistances are high, not because their health pool is absurdly enormous. The effects are most obvious on a high rate of fire gun, such as an automatic scatter laser rifle or high speed minigun, when used on something which can normally take a large amount of abuse, such as a legendary deathclaw. When used by a weapon that fires multiple projectiles per shot, such as a shotgun or a laser with the splitter mod, the Wounding effect applies to each projectile individually, increasing the power of the effect.
 * The bleeding effect also triggers on robots and synths.

Lucky weapon effects

 * Does not stack with Better Criticals perk.

Powerful / Mighty weapon effects

 * Does not work on missile launcher.

Junkie's weapon effects

 * Works on missile launcher.

Console commands
Legendary effects can be added to items through the console by following the next steps:
 * 1) Drop the item on the ground.
 * 2) Pick up the item in the air by holding E (how you would normally drag ragdoll and other objects around).
 * 3) Open console and enter "GetPlayerGrabbedRef." This returns the RefID of the item you're holding in the air.
 * 4) Take note of the returned RefID.
 * 5) Look up the ModID below for the desired legendary weapon effect, note it as well.
 * 6) Enter the following in the console (substitute the appropriate RefID and ModID):
 * 7) For example, you would enter in something like this:.
 * 8) The item will drop to the ground, and when you go to pick it up, you will get an overview similar to the one you get when you pick up any given legendary item for the first time.

As a simpler alternative to points 2 - 4 you can simply left-click on the item when it is lying on the floor. The RefID will then be displayed on-screen just as if you used the mentioned console command. Bear in mind certain weapons can be unusually challenging to left-click when dropped, so if this is unsuccessful, use instead.

You can't stack several mods on a single item, but you can replace a mod with another one via the console.