I wouldn't want DC Hevi 00 for only shooting 20-30 yds as the patterns would be small. They'd give great penetration and good long range patterns, but for your range, I'd go lead.
We pattern our lead buckshot loads at 50yds, some make it out near 70 in the proper set-up. 20-30 would be a chip shot with even #4 from a cyl bore. I shot a doe with 000 at 20 once from a flush mod and it was pretty graphic. All pellets connected and exited.
The best factory patterning buckshot loads we've come across are the Federal Premium buffered copper plated lead #1s. Mod or Impmod usually does the trick.