Originally Posted by
Savage Sniper
there is no reason you couldnt use a 12 gauge for doves. just use lighter loads and if you prefer, a modified choke.
+1 and there is no reason you couldn't use a 20 gauge for ducks or geese.
But to answer your question, no there is not such a gun. Unless someone makes an over under 20 gauge/12gauge. I've never heard of one and would find it almost useless.