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.Magazine.cs

@@ -10,7 +10,7 @@ namespace Content.Shared.Weapons.Ranged.Systems;
 
 public abstract partial class SharedGunSystem
 {
-    protected const string MagazineSlot = "gun-magazine";
+    protected const string MagazineSlot = "gun_magazine";
 
     protected virtual void InitializeMagazine()
     {