I fully agree with King of the Jungle on this one. You do not lunge a horse with a chain in it's mouth. Ever.
I don't like chain bits either, way to harsh. Yes, they do take on the form of the mouth but that doesn't make it gentle. A chain isn't smooth like other bits and therefore causes a lot of discomfort. Rubbing over the gums, which will naturally occur with ANY rider (let alone lunging) isn't really pleasant with a bit that isn't smooth. Plus it gives more pressure than a smooth surface. Image pressing something onto your skin hard, what would be more comfortable, a smooth or a not so smooth surface?

HUP HOLLAND HUP