Changing ammo won't help. A cylinder bore is going to throw a bad pattern (or rather very wide) because it's a cylinder bore.
It used to be popular to install these god-awful, ridiculous, stupid, cumbersome, polychokes onto cut barrels. They were to be the death of many good guns because well meaning but ignorant owners thought they could increase the versatility of their gun. In a situation like yours where you don't care about looks, balance, or sight line, that might be the answer, but I don't know if anyone still makes them.