Joachim
Welcome to AR.
My best loads for my Chapuis 9,3x74R double rifle with 286 grain bullets has been 65 grains of IMR 4831 with the Federal Mag primer.

The Nosler Reloading book shows N140 as the most accurate powder tested. They show the following loads with the 286 Nosler Partition.
51.5 grains = 2132fps
53.5 grains = 2212fps [most accurate load tested]
55.5grains = 2350fps MAX

I have had excellent results on game with the 286 Nosler Partition and the 286 Woodleigh Soft point.
Hope this helps, let us know your results.