Ah! Finally the Nerf we've all been waiting for. I hope this applies to the Vanguard, too! Besides, that Vulcan is all you need Seriously though, looks like a rendering bug. Have you tried relogging? Does the weapon fire if you're inside? Have you even equipped it? You can consider to validate your game files. In the startup menu, before clicking play, select the wrench symbol on the left. Here you will find options to validate your game files, among other things.
It's not a rendering bug.Look this picture,it's from outside the tank And this picture is from inside the tank: