I would start with 257 Roberts loads. Benchmark is a very fast burning powder not really suited for this cartridge. Superformance is on the other end of the burn rate spectrum.Try filling a case with powder and weighing the powder in your DGR case. Compare that to a Roberts case. Use that to estimate how much larger the DGR case is relative to the Roberts case and load accordingly. You're pretty much on your own unless you can find another 257 DGR owner.
I'm another 25 caliber fan and considered a rifle in 257 DGR a while back, but never went ahead and had one built. Good luck with your project.
