Gentlemen, this is very interesting.
I am assuming that the need for a dual diameter boolit is for a bore riding projectile in a rifle with a generous chamber, correct?
Sounds like an issue one would encounter using black in a smokeless chamber (if one wanted a bore riding boolit)
My question is, could the dual diameter be accomplished with the patch alone?
I envision a standard patch with an additional tail which wraps the base of the boolit before the rest of the boolit is wrapped by the full width patch thus increasing the diameter of the base over the diameter of the rest of the boolit. The tail sized appropriately for the amount of neck tension and length of insertion of course.
The paper base may just shear off but the full length outer patch may mitigate that - don't know...
I have a 50/110 that I should try this in.
Something like this: