For example a 20 ga (.614 diameter) projectile in the form of a straight cylinder .614 long would weigh ~520 grains and would require a twist of around 92 inches. And stability would be very questionable. If your not going to use a sabot, as mentioned above, then I suggest you look into a basic design like the regular rifled slugs or miniball, in other words hollow base with a rounded nose so that you can get some length to the bullet and in turn increase the required rate of twist. In addition this would move the center of gravity forward in the bullet and the center of pressure closer to the rear of the bullet, which is natural stability for a projectile in flight.
As to the style of rifling, there are a hundred different schools of thought on that subject. And frankly it weighs much on personnel choice. Personally I would look for one most easily cleaned. Then of course you need to find a finished barrel blank or find a barrel smith who would make it or make it yourself. A daunting project without experience or tooling for such.
Good luck