Short shotguns and short rifles are defined at (c)(1) and (c)(2) respectively; the definitions are essentially the same as federal law. HOWEVER, unlike the feds, California courts have ruled that the length of a rifle with a folding stock is measured with the stock folded, not extended, as the feds do. So a gun that is not a short rifle under federal law may be one under California law. See People v. Rooney, 17 Cal.App.4th 1207 (1 Dist. 1993).
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ATF has created interim rules to implement PL 103-322, and they are a little more specific, and a little more onerous: