I tried with several props with various concavities. All behaved as if the physics engine is working only with the convex hull. Worse, the hull includes the Joint-editor center and end points, which are only for orienting the object's coordinate system and have nothing to do with collisions. Weird. Looks half baked to me.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)