Add missing shotgun modslots & remove double grip slots on MCS (#36389)

* Add missing shotgun modslots, remove duplicate grip on MCS

* Capitalize your acronyms

* Mechanisms

data/json/items/gun/shot.json

@@ -127,6 +127,7 @@
     "clip_size": 8,
     "valid_mod_locations": [
       [ "accessories", 2 ],
+      [ "mechanism", 4 ],
       [ "sling", 1 ],
       [ "barrel", 1 ],
       [ "rail mount", 1 ],
@@ -209,9 +210,12 @@
       [ "accessories", 2 ],
       [ "sling", 1 ],
       [ "barrel", 1 ],
+      [ "grip mount", 1 ],
+      [ "stock mount", 1 ],
       [ "rail mount", 1 ],
       [ "loading port", 1 ],
       [ "brass catcher", 1 ],
+      [ "mechanism", 4 ],
       [ "sights", 1 ],
       [ "underbarrel mount", 1 ]
     ],
@@ -297,7 +301,7 @@
     "copy-from": "remington_870",
     "type": "GUN",
     "name": "Remington 870 MCS",
-    "description": "This Remington 870 Modular Combat system shotgun is currently setup for breaching operations, with a 10 inch barrel and no stock.  It is small enough to carry as a secondary weapon, specifically to pop open any pesky doors you might come across.  The grip's design makes for controllable yet unpleasant recoil, and the barrel lacks any sights.",
+    "description": "This Remington 870 Modular Combat System shotgun is currently setup for breaching operations, with a 10 inch barrel and no stock.  It is small enough to carry as a secondary weapon, specifically to pop open any pesky doors you might come across.  The grip's design makes for controllable yet unpleasant recoil, and the barrel lacks any sights.",
     "weight": "2812 g",
     "volume": "1427 ml",
     "looks_like": "remington_870",
@@ -316,7 +320,6 @@
       [ "bore", 1 ],
       [ "brass catcher", 1 ],
       [ "grip", 1 ],
-      [ "grip mount", 1 ],
       [ "loading port", 1 ],
       [ "mechanism", 4 ],
       [ "rail mount", 1 ],
@@ -364,6 +367,8 @@
       [ "sling", 1 ],
       [ "brass catcher", 1 ],
       [ "barrel", 1 ],
+      [ "stock", 1 ],
+      [ "mechanism", 4 ],
       [ "rail mount", 1 ],
       [ "loading port", 1 ],
       [ "sights mount", 1 ],