JeffG,
Last year I was working with a 355gr LBT/WFN boolit, and for what ever reason did not get the groups I desired.
Doing some reading, asking and talking, I found that some folk in the "KNOW" such as greatly experienced shooters, and the mold maker I was working with said that boolits of over 400gr do tend to do better in the 45/70 then the lighter ones.
So, that tend to go right along with your comments.
Guess we can't say it is a hard fast rule, but when asking what was likely to give me the best cast boolit groups from the 45/70, the answer quickly came back to a couple of over 400 grain slugs, one at 420gr and the other at 465 with the larger having a bit of an edge.
I'm not seeing one hole groups, and may never, but the 465gr slug is out shooting the 355grainer.
The powder, also recommended by the mold maker is H335.
I started this cast boolit critter hunting waaaaaay to late in life to ever gain great amounts of experience.
However, the two I have taken seem to really prove the thoughts about a wide meplat.
My current 465gr cast with it's big wide meplat really did a number on a huge an barren cow elk I took this past Aug. 1st.
Velocity between 1600 - 1700fps, and it really put that critter down in a hurry.
Keep em coming!
Crusty Deary Ol'Coot