i wonder ...
does the ss pins harden the brass
The ss pins act as buffer between the cases while at the same time polishing the cases inside and out. I anneal my brass each time to get consistent neck tension. The annealing process raises the grain of the brass making the neck a little grabby resulting in uneven bullet release and deteriorates accuracy. By using the ss pin after annealing, they polish and clean thereby resolving that problem.
In conclusion, I seriously doubt that the pins harden the brass, but if you tumbled without the pins the casses would collide with one another resulting in peening. Peening is a little lip that forms at the case mouth and that will affect concentricity, neck tension and bullet release.
Kindest regards,
Joe