Since they were dummies, I assume you were still figuring out OAL, crimp, or something similar. Did you try a drop-test with them first with the barrel out of the pistol?
I had this happen once when I was trying out a .38 SPL expander on a Lyman Multi-Expand die in 9mm - the .38 expander did the trick to open the 9mm case up further for the oversize .359 boolits I was using (another story), but when the die was run far enough down to bell the mouth, the expander section of the .38 SPL insert was into the thicker part of the case walls and bulged the walls out. The cases with thicker walls would still chamber if you dropped the slide on them, but would be jammed if you tried to manually extract. You would have to force the slide open on the side of a wood bench.
This was in my earlier days of reloading, and this discovery was made when unloading after the first string of an IDPA match. Embarrassing! These days I always drop-test my ammo first with the barrel out of the pistol.