I don't think anyone was a good shot with that.
That's the rear half of a sight system for the m203; the front half is the fixed post sight on the rifle. This is relevant because, unlike in a game, the shooters POV doesn't change, they put the front element on the target, and adjust the angle of the weapon so that the rear element matches the (horribly) guessed range.
In game, adjusting the angle of the weapon adjusts the shooters POV, meaning that to line up the shot to the appropriate range the numbers need to be inverted.
I hope that makes sense.