J,
Remove the firing pin from the bolt and chamber a standard case. You should feel some resistance when you close the bolt. If not, your chamber is not headspaced correctly and you'll have to form a false shoulder to get the right headspace. With the 257AI, neck down 7X57 brass a little at a time until the bolt closes with with moderately stiff resistance. Keep the bolt lugs lubed.
One old canard on the subject is the need to lube cases. If the brass is pinched in the chamber between the shoulder and bolt face, the brass can only expand radially outward until it hits the chamber wall head on. Lubing cases does nothing useful and can result in oil dents.
Another canard is the need for (relatively) fast burning powders. The difference in time to peak pressure between the fastest and slowest suitable powders is a few milliseconds and makes no difference at all to the final outcome. The important thing is to get enough pressure to form the cases. A maximum charge for the standard 257 Roberts of any powder you have on hand should do nicely.
The standard reloading caveat applies. Dedicate a few cases to working up to maximum. The result you're looking for is a nearly fully formed shoulder. Don't worry if the shoulder/body junction isn't razor sharp. That will come with another firing or three.
