I guess I'd ask, "How far do you plan to shoot something with (I'm assuming" a Forster style slug out of a smoothbore 20 gauge? Personally, I'd probably limit it to 50 or 75 yards and you should be able to compensate with a little "Kentucky elevation".
When you say the rifle barrel is "right on" I'm assuming at 50 yards also? If that's the case, you could carefully take down the front sight a little until the rifle barrel is hitting an inch or two high at fifty, putting it closer to zero at 100 yards or so and reducing the amount the 20 gauge slug is low at 50 yards.
It seems either of those solutions would be preferable to trying to regulate the barrels. Just my $.02
