The design for the bullet is basically the same for both muzzleloader and breach loader if the rifling is the same, but the specs would be different. Daryl is correct in saying the shank (body) of the bullet's lands are going to be made to drop at groove diameter of the barrel for a breach loader. So in other words if the grooves of your barrel were .620" you wan the shank of the bullet to also be .620"

For a muzzleloader the front land is slightly "too big" so it has to engrave when loaded, which keeps it from sliding out if the muzzle is pointed down, but the shank of the bullet should be .001 under land diameter so you can easily load it. You don't want a press fit. Obturation shortens the bullet upon firing. which makes it "fatter" and fills the grooves. In effect the rear of the bullet starts to move forward and sideways before the front of the bullet can, and that's what known as obduration. All soft bullets do this to some extent, but with a breach loader there is far less because at groove diameter, the bullet has no room to upset as it starts.