Audit gun volumes and weights for .50 BMG and improvised guns

This applies the gun volume and weight corrections as of upstream commit
6a34646 .

data/json/items/gun/40.json

@@ -120,8 +120,8 @@
     "name": { "str": "Luty SMG: .40 S&W", "str_pl": "Luty SMGs: .40 S&W" },
     "description": "A Luty pattern makeshift smoothbore SMG crudely constructed out of various steel parts using some of the more advanced powered hand tools; likely one of the most complex guns that are feasible to make outside of a machine shop, but still very unreliable.  This one is chambered for .40 S&W cartridges and accepts custom-made makeshift magazines.",
     "//": "Crafting recipe must make use of angle grinder, bench grinder, set square and vise, which don't yet exist in game. No factory mags until UMP40, mp5/40, or other straight .40 mags are implemented.",
-    "weight": "3274 g",
-    "volume": "1750 ml",
+    "weight": "3520 g",
+    "volume": "1110 ml",
     "price": 20000,
     "to_hit": -1,
     "bashing": 10,

data/json/items/gun/45.json

@@ -200,8 +200,8 @@
     "name": { "str": "Luty SMG: .45", "str_pl": "Luty SMGs: .45" },
     "description": "A Luty pattern makeshift smoothbore SMG crudely constructed out of various steel parts using some of the more advanced powered hand tools; likely one of the most complex guns that are feasible to make outside of a machine shop, but still very unreliable.  This one is chambered for .45 ACP cartridges and accepts MAC-10 compatible magazines.",
     "//": "Crafting recipe must make use of angle grinder, bench grinder, set square and vise, which don't yet exist in game.",
-    "weight": "3274 g",
-    "volume": "1750 ml",
+    "weight": "3520 g",
+    "volume": "1110 ml",
     "price": 20000,
     "to_hit": -1,
     "bashing": 10,

data/json/items/gun/50.json

@@ -7,8 +7,8 @@
     "name": { "str": "Barrett M107A1", "str_pl": "Barrett M107A1" },
     "//": "Price based on unit cost quote for fiscal year 2005 listed at inetres.com, with gunmod modifiers to be added later.",
     "description": "A large, shoulder-fired, .50 caliber anti-materiel rifle.  Its large size, recoil, and noise is offset by its damage and range.",
-    "weight": "12602 g",
-    "volume": "3500 ml",
+    "weight": "12950 g",
+    "volume": "7570 ml",
     "price": 1483300,
     "to_hit": -1,
     "bashing": 12,
@@ -32,7 +32,7 @@
     "name": { "str": "M2HB Browning HMG", "str_pl": "M2HB Browning HMG" },
     "description": "A heavy machine gun used by the US Military from its inception to the Cataclysm, and even rarely by Cataclysm survivors.  Its massive size and design make it impossible to use unless deployed or mounted to a vehicle.",
     "weight": "38000 g",
-    "volume": "5 L",
+    "volume": "9020 ml",
     "price": 1800000,
     "to_hit": -1,
     "bashing": 12,
@@ -93,8 +93,8 @@
     "type": "GUN",
     "name": { "str": "AI AS50" },
     "description": ".50 caliber anti-materiel rifle made by Accuracy International.  With high accuracy for long range target and high fire rate, this weapon is still being used by Greek national guard.",
-    "weight": "13330 g",
-    "volume": "3500 ml",
+    "weight": "12200 g",
+    "volume": "8900 ml",
     "price": 1500000,
     "to_hit": -1,
     "bashing": 13,
@@ -118,8 +118,8 @@
     "type": "GUN",
     "name": { "str": "McMillan TAC-50" },
     "description": "A long-range anti-materiel and anti-personnel sniper rifle made by McMillan Firearms, serving the Canadian Army since 2000 as the C15, and the Navy Seals as the Mk 15 Mod 0.  This .50 caliber bolt-action rifle is capable of defeating light vehicles, radar installations and crew served weapons at extreme distances.  It notably holds the longest range confirmed sniper kill, as well as the 4th and 5th longest.",
-    "weight": "11020 g",
-    "volume": "3127 ml",
+    "weight": "11790 g",
+    "volume": "6500 ml",
     "price": 870000,
     "to_hit": -1,
     "bashing": 12,
@@ -143,8 +143,8 @@
     "type": "GUN",
     "name": { "str": "Serbu BFG-50" },
     "description": "A single-shot, bolt-action rifle made by Serbu Firearms, the BFG-50 is a very affordable firearm for those wishing to shoot .50 BMG.",
-    "weight": "8144 g",
-    "volume": "1387 ml",
+    "weight": "8500 g",
+    "volume": "4000 ml",
     "price": 239500,
     "to_hit": -1,
     "bashing": 12,

data/json/items/gun/700nx.json

@@ -7,7 +7,7 @@
     "name": { "str": "Elephant gun" },
     "description": "A custom-made single shot rifle specially designed for the hunting of huge game.  You could obviously kill everything with this, EVERYTHING.  If you ever find enough ammo of course.",
     "weight": "8082 g",
-    "volume": "3 L",
+    "volume": "4500 ml",
     "price": 2000000,
     "to_hit": 1,
     "bashing": 12,