Cleanup gun containers (probably last time) (#8575)

- snake_case instead of kebab-case
- Remove old MagazineBarrel containers
- Remove old event

Content.Shared/Weapons/Ranged/Systems/SharedGunSystem.ChamberMagazine.cs

@@ -11,7 +11,7 @@ namespace Content.Shared.Weapons.Ranged.Systems;
 
 public abstract partial class SharedGunSystem
 {
-    protected const string ChamberSlot = "gun-chamber";
+    protected const string ChamberSlot = "gun_chamber";
 
     protected virtual void InitializeChamberMagazine()
     {