diff --git a/sim/mage/fire/TestFire.results b/sim/mage/fire/TestFire.results
index 6163d11bfa..13fd0406a6 100644
--- a/sim/mage/fire/TestFire.results
+++ b/sim/mage/fire/TestFire.results
@@ -1692,8 +1692,8 @@ dps_results: {
 dps_results: {
  key: "TestFire-AllItems-RegaliaoftheChromaticHydra"
  value: {
-  dps: 142343.07242
-  tps: 138829.67592
+  dps: 142225.27308
+  tps: 138708.57938
  }
 }
 dps_results: {
diff --git a/sim/mage/fire/critical_mass.go b/sim/mage/fire/critical_mass.go
index 1b08054e9a..a5bef82c5f 100644
--- a/sim/mage/fire/critical_mass.go
+++ b/sim/mage/fire/critical_mass.go
@@ -8,13 +8,22 @@ import (
 
 func (fire *FireMage) registerCriticalMass() {
 
-	getCritPercent := func() float64 {
-		return fire.GetStat(stats.SpellCritPercent) * fire.criticalMassMultiplier
+	getCritPercent := func(isPyroblast bool) float64 {
+		pyroblastCritChance := core.TernaryFloat64(isPyroblast && fire.T15_4pc != nil && fire.T15_4pc.IsActive(), 5, 0)
+		return pyroblastCritChance + ((fire.GetStat(stats.SpellCritPercent) + pyroblastCritChance) * fire.criticalMassMultiplier)
 	}
 
 	criticalMassCritBuffMod := fire.AddDynamicMod(core.SpellModConfig{
-		FloatValue: getCritPercent(),
-		ClassMask:  mage.MageSpellFireball | mage.MageSpellFrostfireBolt | mage.MageSpellScorch | mage.MageSpellPyroblast | mage.MageSpellPyroblastDot,
+		FloatValue: getCritPercent(false),
+		ClassMask:  mage.MageSpellFireball | mage.MageSpellFrostfireBolt | mage.MageSpellScorch,
+		Kind:       core.SpellMod_BonusCrit_Percent,
+	})
+
+	// Separate mod for Pyroblast since it has a different crit bonus from T15 4-piece
+	// which is additive and Critical Mass is multiplicative.
+	criticalMassPyroCritBuffMod := fire.AddDynamicMod(core.SpellModConfig{
+		FloatValue: getCritPercent(true),
+		ClassMask:  mage.MageSpellPyroblast | mage.MageSpellPyroblastDot,
 		Kind:       core.SpellMod_BonusCrit_Percent,
 	})
 
@@ -22,18 +31,21 @@ func (fire *FireMage) registerCriticalMass() {
 		Label: "Critical Mass",
 		OnGain: func(aura *core.Aura, sim *core.Simulation) {
 			criticalMassCritBuffMod.Activate()
+			criticalMassPyroCritBuffMod.Activate()
 		},
 		OnExpire: func(aura *core.Aura, sim *core.Simulation) {
 			criticalMassCritBuffMod.Deactivate()
+			criticalMassPyroCritBuffMod.Deactivate()
 		},
 	}))
 
 	fire.AddOnTemporaryStatsChange(func(sim *core.Simulation, buffAura *core.Aura, statsChangeWithoutDeps stats.Stats) {
-		criticalMassCritBuffMod.UpdateFloatValue(getCritPercent())
+		criticalMassCritBuffMod.UpdateFloatValue(getCritPercent(false))
+		criticalMassPyroCritBuffMod.UpdateFloatValue(getCritPercent(true))
 	})
 
 	fire.RegisterResetEffect(func(sim *core.Simulation) {
-		criticalMassCritBuffMod.UpdateFloatValue(getCritPercent())
+		criticalMassCritBuffMod.UpdateFloatValue(getCritPercent(false))
+		criticalMassPyroCritBuffMod.UpdateFloatValue(getCritPercent(true))
 	})
-
 }
diff --git a/sim/mage/items.go b/sim/mage/items.go
index 9606524fa9..02fe76297b 100644
--- a/sim/mage/items.go
+++ b/sim/mage/items.go
@@ -98,17 +98,13 @@ var ItemSetRegaliaOfTheChromaticHydra = core.NewItemSet(core.ItemSet{
 		4: func(agent core.Agent, setBonusAura *core.Aura) {
 			mage := agent.(MageAgent).GetMage()
 
-			setBonusAura.AttachSpellMod(core.SpellModConfig{
-				Kind:       core.SpellMod_BonusCrit_Percent,
-				ClassMask:  MageSpellPyroblast | MageSpellPyroblastDot,
-				FloatValue: 5,
-			})
+			// Pyroblast is handled in critical_mass.go
+			// Frostbolt is handled in mage.go#ProcFingersOfFrost
+			mage.T15_4pc = setBonusAura
 			setBonusAura.ApplyOnGain(func(_ *core.Aura, _ *core.Simulation) {
 				mage.T15_4PC_ArcaneChargeEffect += 0.05
-				mage.T15_4PC_FrostboltProcChance += 0.06
 			}).ApplyOnExpire(func(_ *core.Aura, _ *core.Simulation) {
 				mage.T15_4PC_ArcaneChargeEffect -= 0.05
-				mage.T15_4PC_FrostboltProcChance -= 0.06
 			})
 
 			setBonusAura.ExposeToAPL(138376)
diff --git a/sim/mage/mage.go b/sim/mage/mage.go
index e9f7eeba0e..a92d8a9756 100644
--- a/sim/mage/mage.go
+++ b/sim/mage/mage.go
@@ -52,11 +52,11 @@ type Mage struct {
 
 	ArcanePowerDamageMod *core.SpellMod
 
-	T15_4PC_FrostboltProcChance float64
-	T15_4PC_ArcaneChargeEffect  float64
-	Icicles                     []float64
+	T15_4PC_ArcaneChargeEffect float64
+	Icicles                    []float64
 
 	// Item sets
+	T15_4pc *core.Aura
 	T16_4pc *core.Aura
 }
 
@@ -91,7 +91,7 @@ func (mage *Mage) ProcFingersOfFrost(sim *core.Simulation, spell *core.Spell) {
 		return
 	}
 	if spell.Matches(MageSpellFrostbolt | MageSpellFrostfireBolt) {
-		if sim.Proc(0.15+core.TernaryFloat64(spell.Matches(MageSpellFrostbolt), mage.T15_4PC_FrostboltProcChance, 0), "FingersOfFrostProc") {
+		if sim.Proc(0.15+core.TernaryFloat64(spell.Matches(MageSpellFrostbolt) && mage.T15_4pc != nil && mage.T15_4pc.IsActive(), 0.06, 0), "FingersOfFrostProc") {
 			mage.FingersOfFrostAura.Activate(sim)
 			mage.FingersOfFrostAura.AddStack(sim)
 		}
@@ -166,7 +166,6 @@ func NewMage(character *core.Character, options *proto.Player, mageOptions *prot
 	mage.HasteEffectsManaRegen()
 
 	mage.Icicles = make([]float64, 0)
-	mage.T15_4PC_FrostboltProcChance = 0
 	mage.T15_4PC_ArcaneChargeEffect = 1.0
 
 	return mage