An old Lyman manual I have for the 2½" .410 with 3/8oz shot recommended 7grs "Infallible" or 10grs "Herco". Infallible was a powder virtually identical to "Unique". In the 2" case using the cut-back plastic wads per 8-bores recommendation I would think 6grs Unique or 8 grs Herco would be very efficient loads for the 3/8oz load. Due to the small dia of the bore Load Density (shot wt/bore area) changes rapidly with only small changes of weight. I would not really think the very slow ".410 Powders" would necessarily be the most desirable for only 3/8oz charges. This Unique loading is often quoted as a suitable charge for loading in .45 Colt case for firing from the SAA incidently.
