You always need to try the fit before getting to deep into casting bullets. I seat the ball on a 6-8" long strip of patching and then pull it back out to test fits.
My 16 bore will shoot WW balls or pure lead but the hard balls need about .010-.012" thinner patches. I use .020-.022 with pure lead.

Lead shot is hardened (chilled) and is as hard or harder than unquenched WW.
I have no idea what rifling form you rifle has but my 16 bore has very narrow lands and very wide grooves. It loads easy. Rifles with wider lands will load much harder with hard lead.
The smaller ball will likely cure the problem
Dan