I've shot 5.0 grains of HP38 under a 158 gr LSWC, hardness approximately 12 BHN.

This is a SOFT load out of a L frame size .357. Feels like a .38.

Since there is data up into the high 6 grains range for other bullets of similar weight (not necessarily lead, though), I decided to go up from 5 grains in half grain increments until I saw some accuracy (I didn't see what I wanted at 5 grains).

5.5 grains, unremarkable grouping
6.0 grains, unremarkable grouping
6.5 grains, able to hold 2 inches at 25 yards off bags.

6.5 grains feels more like a 357, cases fell out, burn was pretty clean. I stopped there.

Next time out I'll try 4.5 and 4.0 grains, going down from 5, and see what happens. I know 4 grains shoots very well in a 38 special case with this bullet.

For magnum level loadings, I've settled on 12.2 grains of AA9. Does every thing I need it to do, doesn't require a magnum primer, and is highly accurate.