I'm looking at the 'halo' and size relationship/space between 'it' and the chairs. Photoshop? The halo is consistent with a photo taken on a white background, meant to have background removed and image placed on top of new background, That or it is designed to shoot around corners
Edit: to add shadows are not consistent either. Camera with flash close to lens is creating nearly no shadow while 'the thing' has a lighting source above and left of lens.
The schematic diagram for the bunker is all in Russian...
That machinegun looks like.... maybe an SG-43 Goryunov?
Make sense, the Goryonov had a quick change barrel, so mounting a custom curved barrel would be fairly straightforward... err... curved forward... whatever. It's late, I should have gone to bed a couple hours ago.
It's that weird cutout on the left side that makes me think its a Goryunov:
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