I got to 33 yards for head/neck killing patterns out of an old black powder SXS that is choked light modified with reloads of 1 3/8 oz of #6 magnum shot, I dropped the powder charge and added a little extra shot trying to tighten up the center core pattern.
I tried #7.5 chilled shot but this gun patterned the #6 magnum shot better
I would think you should get to 40yds with a full or extra full choked gun with out too much trouble.
Some of the old guns were choked pretty tight.