Great points as always, Tony. I should have mentioned in my report that this project builds up on excellent FBI load development work that you, Jim and others have done and posted before.
In fact my favorite powder for this application is same as yours - HS6. First of, it's old, tried and true powder. When you use it, you don't expect unpleasant surprises such as unexpected pressure spikes, excessive temperature sensitivity, etc. There is a lot to be said for using same powder as generations of reloaders used before you.
Based on my pressure calculations HS6 reaches BB velocities at the lowest pressure point among the powders I tested. Of course, calculations are just that - calculations. They could prove inaccurate but that's the best I've got. With 160 gr. soft RB bullet and 2-1/4" SP101 (fast gun with slightly longer barrel than 2" you used) I got following numbers:
6.3 gr. HS6.....843 fps....16,200 CUP (Hodgdon max charge and pressure)
7.0 gr. HS6.....928 fps....19,900 CUP
7.5 gr. HS6.....986 fps....22,800 CUP
8.0 gr. HS6....1042 fps....25,900 CUP
Note that even though BB round is charged with 7.5 gr. of a powder, it is a different powder than HS6. It took 8.0 gr. HS6 to replicate BB velocity. I sent some additional information to your yahoo email address.
Another interesting connection here is that 8.0 gr. HS6 under 158 gr. lead bullet is... That's right, it's the maximum load recommendation for 38 special from Speer #8.
Mike