diff --git a/data/battle_scripts_1.s b/data/battle_scripts_1.s
index 2c0abf1dcc..102cdd7c8c 100644
--- a/data/battle_scripts_1.s
+++ b/data/battle_scripts_1.s
@@ -303,6 +303,56 @@ gBattleScriptsForMoveEffects:: @ 82D86A8
 	.4byte BattleScript_EffectLastResort
 	.4byte BattleScript_EffectRecoil33WithStatus
 	.4byte BattleScript_EffectFlinchWithStatus
+	.4byte BattleScript_EffectRecoil50
+	.4byte BattleScript_EffectShellSmash
+	
+BattleScript_EffectShellSmash:
+	attackcanceler
+	attackstring
+	ppreduce
+	jumpifstat BS_ATTACKER, CMP_LESS_THAN, STAT_ATK, 0xC, BattleScript_ShellSmashTryDef
+	jumpifstat BS_ATTACKER, CMP_LESS_THAN, STAT_SPATK, 0xC, BattleScript_ShellSmashTryDef
+	jumpifstat BS_ATTACKER, CMP_LESS_THAN, STAT_SPEED, 0xC, BattleScript_ShellSmashTryDef
+	jumpifstat BS_ATTACKER, CMP_GREATER_THAN, STAT_DEF, 0, BattleScript_ShellSmashTryDef
+	jumpifstat BS_ATTACKER, CMP_EQUAL, STAT_SPDEF, 0, BattleScript_ButItFailed
+BattleScript_ShellSmashTryDef::
+	attackanimation
+	waitanimation
+	setbyte sSTAT_ANIM_PLAYED, FALSE
+	playstatchangeanimation BS_ATTACKER, BIT_DEF | BIT_SPDEF, ATK48_STAT_NEGATIVE | ATK48_DONT_CHECK_LOWER
+	setstatchanger STAT_DEF, 1, TRUE
+	statbuffchange MOVE_EFFECT_AFFECTS_USER | 0x1, BattleScript_ShellSmashTrySpDef
+	jumpifbyte CMP_EQUAL, cMULTISTRING_CHOOSER, 0x2, BattleScript_ShellSmashTrySpDef
+	printfromtable gStatUpStringIds
+	waitmessage 0x40
+BattleScript_ShellSmashTrySpDef:
+	setstatchanger STAT_SPDEF, 1, TRUE
+	statbuffchange MOVE_EFFECT_AFFECTS_USER | 0x1, BattleScript_ShellSmashTryAttack
+	jumpifbyte CMP_EQUAL, cMULTISTRING_CHOOSER, 0x2, BattleScript_ShellSmashTryAttack
+	printfromtable gStatUpStringIds
+	waitmessage 0x40
+BattleScript_ShellSmashTryAttack:
+	setbyte sSTAT_ANIM_PLAYED, FALSE
+	playstatchangeanimation BS_ATTACKER, BIT_SPATK | BIT_ATK | BIT_SPEED, ATK48_STAT_BY_TWO
+	setstatchanger STAT_ATK, 2, FALSE
+	statbuffchange MOVE_EFFECT_AFFECTS_USER | 0x1, BattleScript_ShellSmashTrySpAtk
+	jumpifbyte CMP_EQUAL, cMULTISTRING_CHOOSER, 0x2, BattleScript_ShellSmashTrySpAtk
+	printfromtable gStatUpStringIds
+	waitmessage 0x40
+BattleScript_ShellSmashTrySpAtk:
+	setstatchanger STAT_SPATK, 2, FALSE
+	statbuffchange MOVE_EFFECT_AFFECTS_USER | 0x1, BattleScript_ShellSmashTrySpeed
+	jumpifbyte CMP_EQUAL, cMULTISTRING_CHOOSER, 0x2, BattleScript_ShellSmashTrySpeed
+	printfromtable gStatUpStringIds
+	waitmessage 0x40
+BattleScript_ShellSmashTrySpeed:
+	setstatchanger STAT_SPEED, 2, FALSE
+	statbuffchange MOVE_EFFECT_AFFECTS_USER | 0x1, BattleScript_ShellSmashEnd
+	jumpifbyte CMP_EQUAL, cMULTISTRING_CHOOSER, 0x2, BattleScript_ShellSmashEnd
+	printfromtable gStatUpStringIds
+	waitmessage 0x40
+BattleScript_ShellSmashEnd:
+	goto BattleScript_MoveEnd
 	
 BattleScript_EffectLastResort:
 	attackcanceler
@@ -397,7 +447,7 @@ BattleScript_CoilDoMoveAnim:
 	attackanimation
 	waitanimation
 	setbyte sSTAT_ANIM_PLAYED, FALSE
-	playstatchangeanimation BS_ATTACKER, BIT_SPATK | BIT_SPDEF | BIT_SPEED, 0x0
+	playstatchangeanimation BS_ATTACKER, BIT_ATK | BIT_DEF | BIT_ACC, 0x0
 	setstatchanger STAT_ATK, 1, FALSE
 	statbuffchange MOVE_EFFECT_AFFECTS_USER | 0x1, BattleScript_CoilTryDef
 	jumpifbyte CMP_EQUAL, cMULTISTRING_CHOOSER, 0x2, BattleScript_CoilTryDef
@@ -3388,6 +3438,10 @@ BattleScript_EffectDoubleEdge::
 BattleScript_EffectRecoil33WithStatus:
 	setmoveeffect MOVE_EFFECT_RECOIL_33_STATUS | MOVE_EFFECT_AFFECTS_USER | MOVE_EFFECT_CERTAIN
 	goto BattleScript_EffectHit
+	
+BattleScript_EffectRecoil50:
+	setmoveeffect MOVE_EFFECT_RECOIL_50 | MOVE_EFFECT_AFFECTS_USER | MOVE_EFFECT_CERTAIN
+	goto BattleScript_EffectHit
 
 BattleScript_EffectTeeterDance::
 	attackcanceler
diff --git a/include/constants/battle.h b/include/constants/battle.h
index 2fedc4aa36..62fcef780b 100644
--- a/include/constants/battle.h
+++ b/include/constants/battle.h
@@ -311,7 +311,7 @@
 #define MOVE_EFFECT_KNOCK_OFF           0x36
 #define MOVE_EFFECT_DEF_SPDEF_DOWN      0x37
 #define MOVE_EFFECT_RECOIL_33_STATUS    0x38
-#define MOVE_EFFECT_NOTHING_39          0x39
+#define MOVE_EFFECT_RECOIL_50           0x39
 #define MOVE_EFFECT_NOTHING_3A          0x3A
 #define MOVE_EFFECT_SP_ATK_TWO_DOWN     0x3B
 #define MOVE_EFFECT_NOTHING_3C          0x3C
diff --git a/include/constants/battle_move_effects.h b/include/constants/battle_move_effects.h
index f9c550318f..d8b1383754 100644
--- a/include/constants/battle_move_effects.h
+++ b/include/constants/battle_move_effects.h
@@ -292,5 +292,7 @@
 #define EFFECT_LAST_RESORT 286
 #define EFFECT_RECOIL_33_STATUS 287
 #define EFFECT_FLINCH_STATUS 288
+#define EFFECT_RECOIL_50 289
+#define EFFECT_SHELL_SMASH 290
 
 #endif  // GUARD_CONSTANTS_BATTLE_MOVE_EFFECTS_H
diff --git a/src/battle_script_commands.c b/src/battle_script_commands.c
index f457fdb86d..c63e1e6d4d 100644
--- a/src/battle_script_commands.c
+++ b/src/battle_script_commands.c
@@ -2606,6 +2606,14 @@ void SetMoveEffect(bool8 primary, u8 certain)
                 BattleScriptPush(gBattlescriptCurrInstr + 1);
                 gBattlescriptCurrInstr = sMoveEffectBS_Ptrs[gBattleCommunication[MOVE_EFFECT_BYTE]];
                 break;
+            case MOVE_EFFECT_RECOIL_50: // Head Smash
+                gBattleMoveDamage = gHpDealt / 2;
+                if (gBattleMoveDamage == 0)
+                    gBattleMoveDamage = 1;
+
+                BattleScriptPush(gBattlescriptCurrInstr + 1);
+                gBattlescriptCurrInstr = BattleScript_MoveEffectRecoil;
+                break;
             case MOVE_EFFECT_RECOIL_33_STATUS: // Flare Blitz - can burn, Volt Tackle - can paralyze
                 gBattleScripting.savedDmg = gHpDealt / 3;
                 if (gBattleScripting.savedDmg == 0)
diff --git a/src/data/battle_moves.h b/src/data/battle_moves.h
index 6c9498ad02..d9370da997 100644
--- a/src/data/battle_moves.h
+++ b/src/data/battle_moves.h
@@ -5493,7 +5493,7 @@ const struct BattleMove gBattleMoves[MOVES_COUNT_GEN6] =
 		.split = SPLIT_STATUS,
 	},
 	{ // MOVE_HEAD_SMASH
-		.effect = EFFECT_PLACEHOLDER, // Needs a custom move effect (1/2 recoil instead of 1/3 recoil)
+		.effect = EFFECT_RECOIL_50,
 		.power = 150,
 		.type = TYPE_ROCK,
 		.accuracy = 80,
@@ -5961,7 +5961,7 @@ const struct BattleMove gBattleMoves[MOVES_COUNT_GEN6] =
 		.split = SPLIT_STATUS,
 	},
 	{ // MOVE_ROUND
-		.effect = EFFECT_ROUND, // Needs a custom move effect
+		.effect = EFFECT_ROUND,
 		.power = 60,
 		.type = TYPE_NORMAL,
 		.accuracy = 100,
@@ -6057,7 +6057,7 @@ const struct BattleMove gBattleMoves[MOVES_COUNT_GEN6] =
 		.split = SPLIT_SPECIAL,
 	},
 	{ // MOVE_SHELL_SMASH
-		.effect = EFFECT_PLACEHOLDER, // Needs a custom move effect
+		.effect = EFFECT_SHELL_SMASH,
 		.power = 0,
 		.type = TYPE_NORMAL,
 		.accuracy = 0,