For comparison sake here's a video of someone shooting the Mg34 in different locations but I don't hear that metallic sound except when the brass hits brass. I don't believe there is any echoing in there either. I'm not saying it isn't correct, I just want to know where they got the metallic sound from.
The thing you are missing is that the video is recorded using a handycam rather than say professional audio equipment. You'll be amazed just how much difference the microphone makes. My bet is that mic has been designed to cut out a lot of top end to help reduce wind noise.
On top of that you are forgetting that the video and audio has been encoded using an algorithm that loses a lot of the sound in order to compress it.
In other words, it's actually very hard to record a gunshot that sounds the same as it does in person, because audio equipment often can't handle the sound pressures involved, and even when it does, your capability of handling those pressures in person might be different to mine.
In other words if ten different people were standing around the gun, they would probably all hear it slightly differently based on a large number of highly variable factors.
Our brain is designed to deal with and compensate for it. That's why we barely notice the difference in sound between shots in slightly different areas, even though looked at as a waveform the sound might be very different.
If you know what to listen for, though, you can tell a lot about where the gun is, by how it sounds when fired.
As someone else noted, audio simulation in games is nowhere near as realistic as video simulation because games designers have taken a long time to realise that we humans actually learn a lot from what we hear without even noticing it.
For example, in the video when the gunner is by the wall and gets shot at point blank by the Russian who came around the wall, I knew he was about to get attacked because I could not only hear the footsteps, but that they were coming from behind him. Zips didn't seem realise that, which is why he almost died. He seemed to look in the opposite direction to where I heard the sound coming from.
Whether that is because he heard it differently, I'll never know. I just know that when I heard those footsteps, if I had been in control, I would have done a 180, then when I saw there was no one behind me on my side of the wall, I would have known exactly where the Russian was.