Brian,
I've said before that as you mentioned in your last post, bullets with a longer bearing surface will generally be more accuracy prone than those with shorter bearing surfaces.
Also, machine cast bullets are very prone to have air inclusions in the bullet, making them unbalanced to a degree, thus hampering fine accuracy. This is not always the case, but can and does happen.
Now, about the BHN and PSI obturation issue. Yes the figures you have quoted are right as rain. And, with bullets that don't properly fit cylinder throats because they aren't sized properly and obturation is necessary for the bullet to seal off powder gasses and prevent blowby and gas-cutting, this can be a real issue. However, it sounds as though your bullets are well mated to your cyinder throats, and obturation isn't an issue here!
We can all get caught running down blind rabbit trails, and spend lots of energy and time going nowhere. I suspect that in your case Brian, with your custom fit throats on the Ruger Bisley, that obturation for accuracy is one of those rabbit trails! The bullet fits from the sounds of things therefore there shouldn't be any gas cutting or blowby. Obturation shouldn't be an issue.
I shoot volumes of our BHN 21 bullets at 700-900 fps with perfect accuracy, and I know that they don't obturate with the loads I'm using! Keep in mind all RNFP profiles aren't created equal, and some don't have a strong front driving band. This difference in designs can make a huge difference in accuracy potential. If you will note the bullets we carry all have very strong front driving bands. There is a reason that we don't make and sell some of the more common bullet designs... they aren't all created equal!
You might try different powders, or perhaps a different crimp. Both can make big differences as can primer selection when it comes to accuracy in these relatively low pressure, low velocity loads. But to chase after different hardness bullets, when what you have fit the cylinder throats so well, I beleive is one of the proverbial rabbit trails.
Yes, I know there are varying schools of thought, and one isn't the definitive answer over the other... I'm just sharing my experiences, such as they are and my very biased opinion.
Don't know whether it helps or not, but have a great day!
God Bless,
Marshall