I have found that any choke larger than "improved cylinder" actually makes patterns worse with #4 through 000 buck shot in my muzzle loading shotguns. I pattern all my smooth bores at 35 yards. I have tried plastic and paper shot sleeves, buffer, Sky-chief loads and "magnum" shot but nothing shoots better than 2, 1/8" over powder card, lubed 3/8 cushion wad and 1 & 1/4 to 1 & 3/8 volume of power to buck shot (volume) and little to no choke.
Eight, 000 buck shot should make a 36" spread at 35 yards in a 12 gauge smoothbore if the powder charge is right. In a 12 gauge I recommend 70 to 80 grains of FFg. Some of my muzzle loading shotguns have removable choke tubes which is a big plus.
Chokes do not appear to improve patterns n buckshot loads in my experience, Plastic shot sleeves, on the other hand, actually do if loaded correctly. I cut the "gas-seal" and "cushion" section off and load over traditional over powder wad and cushion wad for best performance. You have to experiment with your shotgun to find what works best!