Rise time is the primary player certainly. You will note from the picture above that I use card wads? That may be something that supports smokeless/lead obturation. They are cut at .432" diameter and form a fairly tight seal. Previous experience with the 77/44 project using cupped base bullets were not successful to any significant degree. I suspected the base was getting blown by higher pressure as load development progressed, but that is uncertain. There was no leading in any case. As has been observed many time here in the forum, paper patch and nekkid lead kneel to different masters.

Some miscellaneous information from the LASC web page and enumerated in works by Glenn Fryxell:

Bullet BHN / "Minimum" Chamber Pressure For Lead Alloys (PSI)
The formula (from the pages of HandLoader Magazine) to determine at what pressure an alloy of given BHN will obturate the base of the bullet and seal the bore. If the bullet is too hard to obturate, gas cutting usually occurs on the base band on the non-driving side of the rifling and barrel leading is likely. Simply multiply the alloy BHN by 1,422.
Example: Alloy BHN of 12 multiplied by 1422 = 17,064. An alloy of 12 BHN should be used with a load that develops a "minimum" of 17,000 psi. Need more info on minimum / maximum alloy BHN? These Glen E. Fryxell articles explain alloy BHN in easy to understand language.