The 257 STW is so overbore that there are not a lot of guns out there. Load from a Disc is the best way to go to develop handloads. A year ago I chronoed one throwing a 100 grain tsx at 4100 fps from a 29 inch barrel. I don't know what the pressure was. Barrel life must be very short. Surprisingly accuracy was moa. Good luck finding someone to share data.
I believes it's June 1998. The guys I've talked to use 7mm STW data reduced by 15 to 20 percent and work up. Check the books and that might be a place to start--or not...
