I think HuntingSchneider is onto it; it's the 5,6x57 RWS that has the thick necks. I have one and also the .22 WMR and .22 Hornet inserts that are used in it, (and they work well too, by the way. Good for teaching kids to shoot as no recoil).

The 6,5x57 is a Mauser round and I have a Krieghoff Drilling in the rimmed version. Too soon to say much about it, S&B make ammo. I bumped into a gunsmith who'd done a lot of antelope hunting in Africa using one. His hunting mate used the rimmed 6,5x57R version. He was unstinting in his praise of it, particularly using the 93-grain (6 gram) bullet which I presume must have been the RWS one. However on one hunt, he used an H-mantle bullet instead, and shot an antelope from behind that simply ran off and then stopped about 100 meters wondering what on earth was that. The bullet had only created a superficial wound and then glanced off into space. A head-shot sorted it out. I have heard that early H-Mangtle bullets were inconsistent.

I loaded some hoarded 93-grain RWS bullets the other day in an older 6,5x68 RWS custom Mauser by Helmut Bischofer of Austria. 3 of the 5-shot group were touching. Look out chammy's. The 125-grain and 140-grain 6.5mm Nosler partitians are said to work well with the 6,5x57.