Apologies for the late reply, I didn't know there was a response.
I don't think it discriminates between game modes as I think I have noticed it in weekly and in survival, but I haven't played vs. in a while so I don't know. I don't even think I survived being fleshpounded in weekly to have been able to test it in 3rd person, so I could be wrong about that too. I forget stuff. Sorry.
Symptoms are as follows: In 3rd person or watching someone else use the gun, I vaguely hear the sound of the bolt cycling and the subsequent reload that follows the gunshot, but not the gunshot itself.
Uploaded the requested materials as attachments.