Looks too thin from the pic!!! That glass looks paper thin on the top. The bottom looks ok around the hole, but the top (in the pic) is way too thin. Especially if it's breaking while you are taking it off the mandrel. The twisting is putting too much stress on the thin glass. Basically, stack 3 mandrels. That's the absolute thinnest your bead should be when you squish it. (Stack dipped mandrels!!!)
....@ @ <<<<= glass as thick as the mandrel on all sides, no matter the shape.
..@ O @ Bigger mandrel=Bigger glass walls
....@ @
Your bead=
---------- <<< too thin!
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