OK, I know a 257 Roberts is a necked down 7x57 Mauser. I know my beloved 6mm Rem is a necked down 257 Roberts. A 22-6mm Rem and the 224 Texas Trophy hunter (which I think are the same case) are necked down 6mm Rem. So it a 5.6x57 RWS the same as the 22-6mm/224TTH?
From what I remember the case is similar to the other cases, but much thicker especially in the neck which the drawing in the Hornady manual shows as 0.280". A typical .22 centerfire will have a case neck OD around 0.250". The reason I read someplace for the thick neck was that chamber inserts are used in rifles chambered for the 5.6x57 so that it could be shot with less expensive ammunition.
Hornady noted that the necks failed to expand enough to seal the chamber until powder charges were within about 3 gr of max.
hum.... probably would be too fun to turn the neck down on that brass. I have been thinking about a 22-6mm Rem. I currently make my 6mm Rem and 6mm Rem AI from RWS 7x57 brass. It is great stuff and harder than anything else I have ever touched! Might could neck up it and make 6mm Rem brass, by would probably still have to turn a lot off the neck.
I think a much more practical cartridge is the 6X47L.
I have a 6x47L and a 6.5x47L on the want list. However, I currently have a 28" PacNor SuperMatch 1:8 twist that is chambered in a 22-250AI that has quite a few rounds down the tube and I am thinking about what it will be rechambered soon. I don't think I want to set it back and do another 22-250AI. Maybe something different. The 22-250AI is probably overboard. The 224TTH is way overboard. Gives me an excuse to load and shoot.