Little Pete - gotta use a larger sinker. A 00 buck pellet might work for a 30 bore but may not. You will know when you have a tight enough fit.
Hey, it you want to have some fun, let the rifle do the work for you. Load up a few cases with some Unique, maybe 3 gns, add buffer material to keep the powder in place, use sinkers or 00 buck pellets an soft target and mic your recovered slugs.
Whenever I would firelap a barrel I was always interested to note any changes in bore diameter twixt fired and unfired slugs and would regularly recover spent firelapping slugs and measure them. It was fun. HTH. Mikey.