I decided to develop a new 257WBY load and since it is not for targets but for hunting, I wanted to be able to take longer shots with a flat trajectory. So for the last three days I have been working up loads. I decided that I would take any group with my 100gr TSX under a 1/2" as long as I was over 3690fps. I thought it would be great if I could be some where over 3700fps but was not sure that I could get there and be under an 1/2". I used IMR4350, IMR7828, RL-22, RL-25, and Norma MRP. Now I had high expectation for IMR7828, RL-25 and RL-22. I also knew that I could not get velocity out of the IMR4350 but tried it anyway. Now I was able to get under a 1/2" out of all of the powders except MRP but these groups were in the 3645-3674fps range except IMR4350 which was a lot slower. So I loaded hotter charges still with no over max pressure signs as I measure the base of the case and the extreme expansion was .004". Well everything seemed to fall apart and RL-22, RL-25, IMR7828 and IMR4350 accuracy was over an inch and one group over 2 inches. I did not have any confidence in MRP as far as accuracy because in all the years I have used it, I have found that only about 10% of the time does it give me the accuracy I want, but also I knew that MRP would give me high velocities as it does all most all the time. Well, I was surprised to say the least. MRP gave under a half-inch and did it at and average of 3712fps for the three shots with a deviation of only 8fps. I think it could have even been better but I was shooting another rifle to allow my 257WBY between groups to cool. I was shooting a group of 240gr Woodliegh bullets getting 2721fps with the best group being 1 1/4" out of my 30-06 Ackley Imp just before I shot the .458" group using MRP. I am using a 6-18x40mm Buckmaster SF BDC.
OAL was 3.171"