diff --git a/src/main/java/net/fabricmc/loom/util/aw2at/Aw2At.java b/src/main/java/net/fabricmc/loom/util/aw2at/Aw2At.java
index 06fd7e86..e18f5efd 100644
--- a/src/main/java/net/fabricmc/loom/util/aw2at/Aw2At.java
+++ b/src/main/java/net/fabricmc/loom/util/aw2at/Aw2At.java
@@ -37,6 +37,7 @@ import org.cadixdev.bombe.type.signature.MethodSignature;
 import org.gradle.api.Project;
 import org.gradle.api.tasks.SourceSet;
 import org.gradle.api.tasks.SourceSetContainer;
+import org.jetbrains.annotations.VisibleForTesting;
 
 import net.fabricmc.accesswidener.AccessWidenerReader;
 import net.fabricmc.accesswidener.AccessWidenerVisitor;
@@ -80,22 +81,28 @@ public final class Aw2At {
 		remapJar.getAtAccessWideners().addAll(extension.getForge().getExtraAccessWideners());
 	}
 
+	/**
+	 * Converts an access widener file to an access transform set.
+	 *
+	 * @param reader the reader that is used to read the AW
+	 * @return the access transform set
+	 */
 	public static AccessTransformSet toAccessTransformSet(BufferedReader reader) throws IOException {
 		AccessTransformSet atSet = AccessTransformSet.create();
 
 		new AccessWidenerReader(new AccessWidenerVisitor() {
 			@Override
-				public void visitClass(String name, AccessWidenerReader.AccessType access, boolean transitive) {
+			public void visitClass(String name, AccessWidenerReader.AccessType access, boolean transitive) {
 				atSet.getOrCreateClass(name).merge(toAt(access));
 			}
 
 			@Override
-				public void visitMethod(String owner, String name, String descriptor, AccessWidenerReader.AccessType access, boolean transitive) {
+			public void visitMethod(String owner, String name, String descriptor, AccessWidenerReader.AccessType access, boolean transitive) {
 				atSet.getOrCreateClass(owner).mergeMethod(MethodSignature.of(name, descriptor), toAt(access));
 			}
 
 			@Override
-				public void visitField(String owner, String name, String descriptor, AccessWidenerReader.AccessType access, boolean transitive) {
+			public void visitField(String owner, String name, String descriptor, AccessWidenerReader.AccessType access, boolean transitive) {
 				atSet.getOrCreateClass(owner).mergeField(name, toAt(access));
 			}
 		}).read(reader);
@@ -103,16 +110,17 @@ public final class Aw2At {
 		return atSet;
 	}
 
-	private static AccessTransform toAt(AccessWidenerReader.AccessType access) {
+	@VisibleForTesting
+	public static AccessTransform toAt(AccessWidenerReader.AccessType access) {
 		return switch (access) {
-		// FIXME: This behaviour doesn't match what the actual AW does for methods.
+		// This behaviour doesn't match what the actual AW does for methods.
 		//   - accessible makes the method final if it was private
 		//   - extendable makes the method protected if it was (package-)private
-		//   Neither of these can be achieved with Forge ATs without using bytecode analysis.
-		//   However, this might be good enough for us. (The effects only apply in prod.)
+		// It also ignores that mutable doesn't change the visibility (but AT disallows bare "-f" modifier).
+		// None of these can be achieved with Forge ATs without using bytecode analysis.
+		// However, this is good enough for us. (The "unwanted" effects to visibility only apply in prod.)
 		case ACCESSIBLE -> AccessTransform.of(AccessChange.PUBLIC);
-		case EXTENDABLE -> AccessTransform.of(AccessChange.PUBLIC, ModifierChange.REMOVE);
-		case MUTABLE -> AccessTransform.of(ModifierChange.REMOVE);
+		case EXTENDABLE, MUTABLE -> AccessTransform.of(AccessChange.PUBLIC, ModifierChange.REMOVE);
 		};
 	}
 }
diff --git a/src/test/groovy/net/fabricmc/loom/test/unit/forge/Aw2AtTest.groovy b/src/test/groovy/net/fabricmc/loom/test/unit/forge/Aw2AtTest.groovy
new file mode 100644
index 00000000..12d7707c
--- /dev/null
+++ b/src/test/groovy/net/fabricmc/loom/test/unit/forge/Aw2AtTest.groovy
@@ -0,0 +1,67 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2022 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.unit.forge
+
+import net.fabricmc.accesswidener.AccessWidenerReader
+import net.fabricmc.loom.util.aw2at.Aw2At
+import org.cadixdev.at.AccessChange
+import org.cadixdev.at.ModifierChange
+import spock.lang.Specification
+
+class Aw2AtTest extends Specification {
+	def "test accessible"() {
+		when:
+			def at = Aw2At.toAt(AccessWidenerReader.AccessType.ACCESSIBLE)
+
+		then:
+			at.access == AccessChange.PUBLIC
+			// AW makes previously private methods also final, but this cannot be replicated
+			// using Forge AT.
+			at.final == ModifierChange.NONE
+	}
+
+	def "test extendable"() {
+		when:
+			def at = Aw2At.toAt(AccessWidenerReader.AccessType.EXTENDABLE)
+
+		then:
+			// AW makes previously private methods protected and does not change the visibility
+			// of protected methods, but this cannot be replicated using Forge AT.
+			at.access == AccessChange.PUBLIC
+			at.final == ModifierChange.REMOVE
+	}
+
+	def "test mutable"() {
+		when:
+			def at = Aw2At.toAt(AccessWidenerReader.AccessType.MUTABLE)
+
+		then:
+			// The access change to public is needed because the Forge AT format cannot
+			// have a bare "-f" modifier. Reusing the previous visibility needs bytecode analysis,
+			// so this is good enough.
+			at.access == AccessChange.PUBLIC
+			at.final == ModifierChange.REMOVE
+	}
+}
diff --git a/src/test/resources/projects/forge/aw2At/expected.accesstransformer.cfg b/src/test/resources/projects/forge/aw2At/expected.accesstransformer.cfg
index 493714c9..ed4649a7 100644
--- a/src/test/resources/projects/forge/aw2At/expected.accesstransformer.cfg
+++ b/src/test/resources/projects/forge/aw2At/expected.accesstransformer.cfg
@@ -1,2 +1,3 @@
 public net.minecraft.world.level.GameRules$BooleanValue m_46250_(Z)Lnet/minecraft/world/level/GameRules$Type;
+public-f net.minecraft.world.level.storage.loot.LootTable f_79108_
 public-f net.minecraft.world.level.block.IronBarsBlock m_54217_(Lnet/minecraft/world/level/block/state/BlockState;Z)Z
diff --git a/src/test/resources/projects/forge/aw2At/src/main/resources/my.accesswidener b/src/test/resources/projects/forge/aw2At/src/main/resources/my.accesswidener
index a9babdd7..2f7216be 100644
--- a/src/test/resources/projects/forge/aw2At/src/main/resources/my.accesswidener
+++ b/src/test/resources/projects/forge/aw2At/src/main/resources/my.accesswidener
@@ -1,3 +1,4 @@
 accessWidener v1 named
 accessible method net/minecraft/world/level/GameRules$BooleanValue create (Z)Lnet/minecraft/world/level/GameRules$Type;
 extendable method net/minecraft/world/level/block/IronBarsBlock attachsTo (Lnet/minecraft/world/level/block/state/BlockState;Z)Z
+mutable field net/minecraft/world/level/storage/loot/LootTable paramSet Lnet/minecraft/world/level/storage/loot/parameters/LootContextParamSet;