jwp475, Thank you for that information on WC872.
At the moment my maximum load is 69.0 grains/H870 with the 156 grain Wildcat in my 257Weatherby. This represents a load density of around 81%, and I would prefer to use a ball powder that achieves at least 85% and preferably over 90%.
My early trials using Vihtavouri 20N29 have been a flop, with powder bridging seeming to be the cause of my problems. I will try some lighter loads soon, just in case this lot of 20N29 is much faster burning than I expected. Brian.