Welcome to the forum. You will probably be better off shooting .535" diameter RBs from that Smoothbore, with a good patch of at least .015" thickness, rather than trying to shoot a .550" RB. Unlike any rifle, there is NO place for the fabric to Squeeze into( grooves) in a smoothbore, and you need more "room " between the ball and the side of the bore for your patching, than you might need in a rifled barrel of the same diameter.
I join others in suggesting that you go to the Links Section, which you can find under " Member Resources" on the index page to this forum. Scroll down to "Articles, Charts, and Links", and click on the links. It will take you to a table of contents that lists LOTS of suppliers by name. Click On Track of the Wolf( TOTW, or sometimes TOW)and it will take you to the company website, and catalog. Look for "Ammo", then cast bullets and balls, and you will find the diameter of ball you seek. These are sold in quantities of 50, so that you can test your gun to determine if you can get good accuracy using that diameter ball, BEFORE investing in a mold.
A lot of members have the same gun, and use the .535" ball with thicker patches. For Hunting Eastern Whitetails, the gun and ball have enough "Oomph" to get the job done nicely, if you do your part. Accuracy varies from gun to gun, with just about everyone getting fine accuracy out to 50-60 yds, and some finding good accuracy on out to 100 yds. Most shots, however, are taken at deer standing Less than 50 yds from the shooter.
Good luck to you. :thumbsup: