My reloading efforts with the 257 Weatherby Magnum are really only just getting started. On acquiring my Weatherby Vanguard S2 rifle a few months ago I had a single packet of factory 100 grain SP loads. That was enough for me to sight the rifle and go hunting; successfully for wild dogs as it transpired.
I also had a box of 50 Nosler cases for the 257 Wby which I have done some initial handloading with. For that I used Reloader 25 propellant. Naturally, I started at recommended charge levels and worked carefully up towards maximum loads, keeping an eye out for any signs of excess pressure.
Given that the 257 Weatherby Magnum is a hot, overbore calibre that consumes impressive amounts of propellant I felt it wise to keep a close track of case dimensions. I used my callipers to establish that the unfired cases were a very consistent 2.540 inches in length.
Similarly, the diameter of the case belt was 0.530 inches as measured with my callipers. To be sure about that I went to the trouble of remeasuring the case belt diameters with my more accurate micrometer. That showed a consistent 0.5297 inch measurement.
Calliper measurements of the case belt at the range indicated a 0.001 to 0.002 inch increase on initial firing. Today I took 20 of the cases that have been fired three times now and ran the micrometer over them. That showed an average diameter of 0.5315 inch. That expansion of 1.8 thou seems quite reasonable to me as it duplicates the 1.9 thou expansion I measured in the factory rounds. In future load development I will mic each case before and after firing to be better informed of actual expansion values.
When reloading the 257 Weatherby I do a gentle full length resize on each case. I have read that the Weatherby Magnums can exhibit case lengthening after as little as three firings. While I did not suspect I should be in that territory just yet I decided to check the length of my thrice fired cases after I had micced the belt diameters. That gave me a surprise. All twenty cases had lengthened and were in need of a trim. I used my case trimmer to bring them all back to 2.540 inches. I will now routinely check case length at each reloading.
In my rifle, with its 24 inch barrel, I found that 75.0 grains of RL25 produced an average muzzle velocity of 3,390 feet per second with the 100 grain Nosler Partition.
Likewise, 73.0 grains of RL25 delivered an average MV of 3,150 fps with the 110 grain Nosler Accubond. The few shots I have taken at game with the 110 AB delivered emphatic results with outstanding terminal ballistic performance.
For both the Partition and Accubond loads accuracy was acceptable at about the MOA mark. I expect to improve on that as the barrel wears in and I refine my loads.
However, further supplies of Reloader 25 are unavailable so that prompted a move to the ADI powder AR2217, which is in plentiful supply here in Australia. I will be testing that propellant behind the Nosler projectiles in the coming weeks.
There is only limited data available for reloading the 257 Weatherby Magnum with AR2217. I was obliged to interpolate the loads for the 110 grain Nosler Accubond from ADI data for projectile weights on either side of that bullet weight.