How far do you want to shoot with it? A smooth bore will limit your range in most cases. Probably a max of 75 yards is your lucky, and the groups will be marginal at that. A rifled barrel should get you at least 3 inches at 100 yards, usually better in most cases.

I personally don't see where you would need an extra barrel if it was a smooth bore. Unless the bird barrel were a fixed full choke.

Paul