Use define for iron ball weight increment

include/hack.h

@@ -29,6 +29,9 @@
 #define EXT_ENCUMBER 4 /* Overtaxed */
 #define OVERLOADED 5   /* Overloaded */
 
+/* weight increment of heavy iron ball */
+#define IRON_BALL_W_INCR 160
+
 /* hunger states - see hu_stat in eat.c */
 #define SATIATED 0
 #define NOT_HUNGRY 1

src/objnam.c

@@ -3533,7 +3533,7 @@ typfnd:
     }
     otmp->owt = weight(otmp);
     if (very && otmp->otyp == HEAVY_IRON_BALL)
-        otmp->owt += 160;
+        otmp->owt += IRON_BALL_W_INCR;
 
     return otmp;
 }

src/read.c

@@ -2271,7 +2271,7 @@ struct obj *sobj;
         You("are being punished for your misbehavior!");
     if (Punished) {
         Your("iron ball gets heavier.");
-        uball->owt += 160 * (1 + sobj->cursed);
+        uball->owt += IRON_BALL_W_INCR * (1 + sobj->cursed);
         return;
     }
     if (amorphous(youmonst.data) || is_whirly(youmonst.data)

src/weapon.c

@@ -304,12 +304,12 @@ struct monst *mon;
     if (ptr == &mons[PM_SHADE] && !shade_glare(otmp))
         tmp = 0;
 
-    /* "very heavy iron ball"; weight increase is in increments of 160 */
+    /* "very heavy iron ball"; weight increase is in increments */
     if (otyp == HEAVY_IRON_BALL && tmp > 0) {
         int wt = (int) objects[HEAVY_IRON_BALL].oc_weight;
 
         if ((int) otmp->owt > wt) {
-            wt = ((int) otmp->owt - wt) / 160;
+            wt = ((int) otmp->owt - wt) / IRON_BALL_W_INCR;
             tmp += rnd(4 * wt);
             if (tmp > 25)
                 tmp = 25; /* objects[].oc_wldam */