The 5.6x57 isn't so popular because it has a rather thick neck in order to accommodate the used of sub caliber inserts. RWS brass can be hard to find at times and due to the large neck, reforming 7x57 brass leaves you with a sloppy neck.
The 8x57 and 7x57 are different cases. Yes, they are 57 mm long but otherwise very different.
People tend to assume that you can simply neck up 8x57 brass and have 9x57 or 9,3x57 brass. yes, it will fireform but it is not correct by simply necking up. I load and shoot all of the above and enjoy them all.
