YOu can buy a commercial Buffer, called Puff-Lon that is finer than the corn mix, and will also buffer the shot. Anything you do that protects the shot from distorting when the gun is fired will produce tighter patterns. A plastic Cup, made for steel shot, together with a buffer is probably the best combination of things to use in cylinder bore guns to improve patterns.
All this is expensive, NOT Historically Correct, and therefore objectionable to many shooters here. But if you want to play around with stuff like this, it can work.
I prefer to think of a cylinder bore gun as a 25 yard hunting gun, and pass on longer shots. I don't always because I find, as most hunters, that I pick up a bird that may be 15-20 yards out, but by the time I get my barrel in front of it, the bird may be at 30 yards or a little further when I fire my gun, and is definitely further when the shot reaches it. My use of #5 shot has brought down birds that would have been missed or crippled with smaller shot.