diff --git a/src/robotwar/register.clj b/src/robotwar/register.clj
index 14a3eaf..0755fdd 100644
--- a/src/robotwar/register.clj
+++ b/src/robotwar/register.clj
@@ -123,7 +123,8 @@
                                              GAME-SECONDS-PER-SHOT)]
              (assoc 
                world-with-new-shot-timer
-               :shells (merge shells (shell/init-shell pos-x pos-y aim next-shell-id data))
+               :shells (merge shells {next-shell-id
+                                      (shell/init-shell pos-x pos-y aim data)})
                :next-shell-id (inc next-shell-id))))))})
 
 (defn get-target-register
diff --git a/src/robotwar/shell.clj b/src/robotwar/shell.clj
index cb5e0d4..d7e3e56 100644
--- a/src/robotwar/shell.clj
+++ b/src/robotwar/shell.clj
@@ -3,12 +3,11 @@
   (:require [robotwar.physics :as physics]))
 
 (defn init-shell
-  [pos-x pos-y aim id distance]
+  [pos-x pos-y aim distance]
   ; TODO: make the starting point dependent upon the robot radius,
   ; which should be in constants.
   (let [{unit-x :x unit-y :y} (physics/decompose-angle aim)] 
-    {:id id
-     :pos-x pos-x
+    {:pos-x pos-x
      :pos-y pos-y
      :v-x (* unit-x SHELL-SPEED)
      :v-y (* unit-y SHELL-SPEED)
diff --git a/src/robotwar/world.clj b/src/robotwar/world.clj
index f57ed92..06afd05 100644
--- a/src/robotwar/world.clj
+++ b/src/robotwar/world.clj
@@ -29,9 +29,17 @@
                     (robot/tick-robot (robots robot-idx) world))
                   starting-world
                   (range (count (:robots starting-world))))
-        ticked-shells (map shell/tick-shell shells)
-        live-shells (remove :exploded ticked-shells)
-        exploded-shells (filter :exploded ticked-shells)]
+        ticked-shells (zipmap (keys shells)
+                              (map shell/tick-shell (vals shells)))
+
+        ; TODO: Is this the most idiomatic way to map a hash-map, testing
+        ; each value in the hash-map on a predicate and then returning another hash-map? 
+        ; looks clunky.
+
+        live-shells (into {} (remove #(:exploded (val %)) 
+                                     ticked-shells))
+        exploded-shells (into {} (filter #(:exploded (val %))
+                                         ticked-shells))]
     ; TODO: make this a real let-binding, that determines
     ; which robots were damaged.
     (let [damaged-world ticked-robots-world]