Miller, I don't recall what source chart I took thes shot size from, but I could easily have gotten it wrong. However, I think you get the idea of what a bridge might look like within the shot population. Whereas the shot generally flows freely forward at accelerating velocity as it passes the choke, if a bridge exists, the two abutment pellets would jamb upon choke constriction entry. Sufficient force would have to be generated to collapse the bridge. Collapse could occur from slight misalignment or deformation of the pellets. I think the probabilities for alignment necessary to form a strong bridge is low. Hence, riveling is uncommon.

I'm going to LV gunshow for the weekend and will revisit this thread next week.

DDA