diff --git a/bridge/sds/Observation.java b/bridge/sds/Observation.java
index cb45bb6..49bbe06 100644
--- a/bridge/sds/Observation.java
+++ b/bridge/sds/Observation.java
@@ -419,6 +419,18 @@ public final class Observation {
             ObjectNode wn = weapons.addObject();
             wn.put("id", e.getEquipmentNum(w));
             wn.put("name", wt.getName());
+            // Where it is bolted on. An arm gun, a torso gun and a leg gun do
+            // not bear on the same hexes: Mek.getWeaponArc reads the mount, and
+            // Mek.isSecondaryArcWeapon is false for the legs, so a leg weapon
+            // fires forward and stays put when the torso twists. Sent as
+            // MegaMek's own abbreviation, the same key "locations" uses.
+            wn.put("location", e.getLocationAbbr(w.getLocation()));
+            // Rear-mounted beats the location outright - the weapon fires into
+            // the rear arc wherever it sits. Sent only when true, like
+            // "destroyed" on a location.
+            if (w.isRearMounted()) {
+                wn.put("rearMounted", true);
+            }
             wn.put("heat", wt.getHeat());
             wn.put("short", wt.getShortRange());
             wn.put("medium", wt.getMediumRange());
diff --git a/docs/PROTOCOL.md b/docs/PROTOCOL.md
index 178eba3..dca0426 100644
--- a/docs/PROTOCOL.md
+++ b/docs/PROTOCOL.md
@@ -77,7 +77,8 @@ the board is a property of the *unit*, which is why `movementMode` and
             "locations": [{"name": "CT", "armor": 20, "armorMax": 22,
                            "rearArmor": 6, "rearArmorMax": 8,
                            "internal": 18, "internalMax": 18}],
-            "weapons": [{"id": 3, "name": "LRM 10", "heat": 4,
+            "weapons": [{"id": 3, "name": "LRM 10", "location": "RT",
+                         "heat": 4,
                          "short": 7, "medium": 14, "long": 21,
                          "rackSize": 10, "damagePerPacket": 1.0,
                          "usable": true,
@@ -95,6 +96,16 @@ and side torsos. Every number is a count; MegaMek's negative armour states
 converted to zero, and a location that is gone carries `"destroyed": true`
 instead. A location the unit does not have is left out.
 
+A weapon's `location` is where it is bolted on, by MegaMek's own abbreviation —
+the same names `locations` uses. The mount is what decides which hexes a weapon
+bears on: `Mek.getWeaponArc` reads it, and `Mek.isSecondaryArcWeapon` is false
+for the legs, so a leg weapon fires along the unit's facing and a torso twist
+does not move it while an arm or torso weapon follows the twist. `rearMounted`
+is sent only when true and beats the location outright: the weapon fires into
+the rear arc wherever it sits. A weapon with no `location` — an observation
+recorded before the field existed — is read as a centre-torso weapon, which
+fires forward and follows the twist like every mount but the arms and the legs.
+
 `ammo` is what is in the tube, and is absent for an energy weapon and for an
 empty rack. A launcher's damage is not a property of the launcher: an SRM-6
 throws two points a missile with standard ammo and none with Inferno, which
diff --git a/plan/protocol.md b/plan/protocol.md
index 879c2b2..d4db8d6 100644
--- a/plan/protocol.md
+++ b/plan/protocol.md
@@ -27,6 +27,13 @@ Adding a field is deliberate, with a reason in the commit message.
       cannot be right without it. On the wire as a weapon's `ammo`.
       `damagePerPacket` is still computed from the launcher alone; making it
       read the munition belongs to [ev](ev.md)
+- [x] **Mount location per weapon** - which weapons bear from a candidate
+      position depends on where they are bolted on, and the wire carried ranges
+      and damage and nothing else. On the wire as a weapon's `location`, in
+      MegaMek's own abbreviations, with `rearMounted` beside it. The case that
+      makes it load-bearing is the legs: `Mek.isSecondaryArcWeapon` is false for
+      them, so a leg weapon does not follow the torso twist, and a candidate
+      generator that assumed a torso mount would overvalue twisting
 - [ ] **Per-unit levels in the round tick** - on the match result as
       `entities`, `{seat: {entity id: {bv, armor, internal, potential, force}}}`,
       beside the side totals it is the per-unit form of. A destroyed unit drops