I think you will need to pattern the guns, but my choice would be the 12 gauge with modified choke and a hard shot comparable to Remington Hevi-Shot loads. I think you'd be able to get a decent tight pattern at 40 yards with it. If I only had the full choke 20 gauge I'd use relatively small lead shot, about #6. But I'd pattern it before using it. 40 yards is a long shot with a 20, even with a full choke.