Check your chamber throat diameter. Newer Smiths (and the Charter I just bought)
have close to SAMMI spec .429 throats.
My older (1980s) Smiths have much larger throats going up to
.432. You best accuracy and least leading with cast bullets will
be with bullets matching the throat diameter.
To get on subject here, I have recently been on a campaign
to obtain a good load for my 3" 624. I have finally found at least
one as of last week. It consists of the .431 (actually measures
.432) Laser-Cast 240 grain SWC from Oregon Trail and 7 grains of
Universal (Clays) powder in a Starline case (these are nickel plated).
7.5 grains performs almost as well. Typical Skeeter Standard Load

I use a CCI 300 primer.
6 grains of Unique with a 240 SWC is the generic low power target
load in all my 44's.
Getting a load to shoot well in the 4" 624 is easy. This is the best one
so far in the 3"er which does tend to scatter 'em around a bit more. Target is at 30 feet.
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Nemo