SEPIA , Before you go off half cocked shoot the rifle with 286 gr bullets! I have a model 140E 9.3X74R Merkel double rifle, that was regulated with the same bullet yours is, according to the paper work, but it shoots 286 gr bullets just fine! If the rifle you just bought is a model 141, then it has an adjustable regulation, and can be adjusted to any bullet weight you want.

With the example CptCurl was talking about with his gun shooting "wide" could possibly have been fixed by speeding the velocity up slightly to bring the two barrels together! I'd bet my hat, his wide barrel groups were high as well as wide, and a slight increase in speed would likely have brought them together, and lower at the same time.

Start with 66.0 grs of H-4831m with a 286 gr Nosler Partition, and work up slowly to a max of about 68 grs of H-4831. the ranges are between 2119 fps/ 33,400 CUP, to 35,800 CUP/ 2200 fps. I think you will find your sweet spot someplace in that range, then substitute other makes of bullets of the same weight, to see if they will all shoot well. If you can work up a load that shoots to the regulation with a 286 gr bullet without going to the considerable expense of re-regulating, you are money ahead. Not only that, but to get the best from your double handloading is must!

Good luck, and enjoy the affiction you have just contracted, because it is incurable! You blood tests will forever, from this day forward, show that you are DR positive! (Double Rifle positive)