Well after trying AR2209, AR2213SC, AR2217 & R15 in my Merkel 140AE
Best powder ended up R15 using 66gn
Woodleigh 400gn SP, 215Federal primer, 20mm backing rod to sit on the powder to take up air space Approx 2000fps
Grouping 42mm Wide / 55mm High at 50m
Certainly no Holland & Holland or a Rigby.
Tried the Hornady factory loads about 1937fps for those, a lot slower than expected.
2 shots mimicked factory test results.
The lower speeds in all the other powders the groups were not good to say the least.
Rather finicky but a lot of fun.