Good Afternoon C. J.
You made some very good points, along with good observations.
Everything in the shooting game is a trade off or compromise; regardless if it is PRB, conicals, or centerfire (black or smokeless powder).
Your experience with the Douglas 36 caliber 1-66" rifling is a first class example, and you are quite correct in youR findings.
The 1-66" 36 caliber would have performed better with a heavier powder charge. That would have increased the velocity to the higher level needed to stablize the round ball for that particular rifling twist. BUT THAT IS NOT what you wanted.
In this case, 1-48" rifling twist with .008" - .010" deep rifling grooves would have given you the performance you wanted from a 36 caliber rifle; good accuracy with low powder charges.
At the risk of repeatedly sounding repetitive, I touched on this in my long post on accurate barrels.
Of the 25 plus T-C,s I have shot and worked with since their introduction (no, I did not own all of them, only 2), the consistently most accurate ones were the 45 caliber and smaller bores. The T-C. 1-48" rifling twist with the .006" - .007" is more suited to those smaller bores.
One of the Forum members is proving that with some very fine shooting with a T-C 40 caliber.
Of course every time, I post something like this, that is half critical of the 1-48" rifling twist, I receive much criticism.
However, post like yours tends to re-inforce the experiences of me and many other older more experienced black powder shooters.
Keep up the good shooting and evaluations.
As usual, my opinions and procedures on this subject can be accepted or rejected, used or not used.
Best regards and good shooting,
John L. Hinnant
If you are not an NRA Member, why not? I am carrying your load.