It's just a small Texture Bug in the Texture of the big stationary Rocket Launcher. The middle barrel has no inner texture so you can see "Through it".
A Screenshot is in the Attachments.
This has been around for longer then a year now.
I just reported it and it will be eventually fixed. We will update the topic once we have more informations.
Keen Software House: QA Department
Voting for 2 here.
I noticed this yesterday & it's since been reported by another on the Steam forum (additional screenshot in the link if it's helpful)...
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