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OK, this is where things have gone from the frustrating to the obsessive and back again.
Range testing the 15.9 grain load revealed a good group for three of the five shots of about an inch - two touching one out. The other two were six inches vertically low. So this is the same grouping as from the start and indicates the extreme velocity spread (ES) is much too wide.
So what? This is where it gets technical and goes straight to the heart of why Whispers/subsonic loads are so frustrating. Let's see if I can get the description right. The pressure curve in the case is not uniform because the powder is not burning at the same rate from shot to shot. Because the powder is not burning at the same rate from shot to shot, the gas pressure is different from shot to shot and the velocity ranges high and low - which is what you'll see in the velocity spikes in the records detailed above.
It is blindingly obvious in hindsight but the sort of thing you need external advice on. It was also the conclusion that I was coming to after having standardized the load, the seating depth, the velocity etc etc.
The answer is that if you are seeing perfectly vertical groups go to a faster burning powder. With the BR case size you need to look at something in the burn range of IMR4227, H4227 or AA1680 ie a powder that suits both rifle and pistol, very fast so that it can used in a 22 Hornet and also a magnum pistol case.
Back tot the drawing board/reloading bench. My ideal load will have the following characteristics when using a 240 grain SMK.
Velocity: 1045fps
SD: 11 fps
Group at 100: about an inch and hopefully less.
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