I will say it here also. I think this gun is a recent rebore of generic 12.9 cartridge of some sort.

I have a 577/500 No.2 Express on a Greener Martini action. It is a beast to shoot. I would not want to shoot this lighter gun in its current stock and barrel configuration, it would be unpleasant.

As to safe, this appears to be rifle action and that has passed recent proof, I see no reason to not shot it.