It's not so much that the definition is hazy, as that it is arbitrary. And it's purely a legislative definition, so it's actually very easy to legislate against. It's much harder to explain how the resulting legislation is coherent or rational.
For example, a Ruger Mini-14 is the same caliber and has the same core functional features as an AR-15. Any self-styled commando wishing to assault a bank or a police station or a meth house or their ex-girlfriend's boyfriend's Malibu beach compound would do just as well with either one. Any soldier needing an assault rifle could pick up either one and tick that box.
But the latter is banned as an "assault weapon" in California*. The former is not. Indeed, there are many light hunting rifles and "varmint" rifles, that are functionally identical to the AR-15, that are allowed in California. If the legislators behind this ban had a sincere concern for the threat to public safety posed by these weapons, they would have banned all the variations. Not just the one. If I were a judge, I'd be inclined to overturn the ban on that basis alone.
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*Unless this new ruling is upheld.