Daryl,
I managed to review the majority of Greener's 9th Ed. over the past two days, and it now has MORE note-tags at the top of its pages - even tho it was already the most annotated and underlined book in my whole library of gun books!

I found the part where Greener shows his remarkable "shadow-graphs" of fired SG loads. He does mention that his interpretation of what is happening at the muzzle could be conjecture, but says it explains what appears to be happening with a choked load vs. one from a cylinder bore. I'm quite sure I've seen modern high-speed photographs that show it is not the wad pushing up thru the shot column that is causing a doughnut pattern. But again it is likely a choked muzzle was involved for the modern pics.

So I suspect both issues are correct, helping to explain what a troublesome issue doughnutting can be, and both gas activities at the muzzle and/or wad effects can be involved in that discouraging pattern effect.

I noted again that Greener really emphasizes the necessity of using a good-quality felt wad for SG loading, with a hard card beneath that to seal the lubed felt from the powder. He also thinks putting a thin card atop the felt wad is probably not necessary. I like the idea of making a "cookie" out of the lubed felt wad tho, using a card wad both below and above it, and think it makes it into a better gas-piston and bore-wiper.

I didn't find the part about considerably roughing up the muzzle end of the bore to retard the wads like a choke does. Can that be elsewhere - W. Greener maybe?

Aloha, Ka'imiloa