Hi

I use a 105gr SWC slug cast from range scrap with 3.4gr of Bullseye or HP38. Very light bullet, makes very neat holes in paper and very accurate at 20yds, haven't tried any further out. Works great with the same load in my Marlin and Alfa LBR, saves me making two different loads.
I have to cast the bullets myself and as they are a SWC profile, they are rubbish for rapid feeding from the Marlin tube magazine, so I just load one at a time.
I have used less powder before - 3gr was my standard load for a while - but found I preferred a slightly louder bang with 3.4gr. Couldn't see any difference in accuracy though that's my dodgy eyes and iron sights rather than proper load development.

Cheers!