Not quite sure why nobody suggested this yet, but you can make a 1:1 mirror with 2d scene capture actor.
Bear in mind that, there’s no material function to blur the texture sample as of now, so getting the reflection to render without jaggies will require quite big resolution.
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Actually now i see that it has been suggested.
Well, this is as good as it gets for now. With right material setting it is possible to set up 2d RT for a mirror with around 60 to 100% satisfaction rate, depending on usage.
I’ve got it to work very well, but only at bigger than up-close distances and angles weren’t clamped, so if player was looking at a low angle it would just stretch the texture.