As long as you realise you are conducting a test , and always confirm impact on target , and check the bore, there is no harm in sticking a lead bullet. Firing the next shot into an obstructed bore is a Bad Thing .once you find that point , increase at least .1 , preferably .2gr , or at least 25fps by the chrono.
And just to show how variable guns and ammunition are I used an identical charge as Mr Surveyer , and had markedly different result. ( Since the merger , it has been publicly stated that among other combos , that HP-38 is exactly the same powder as WW-231 , bottled from the same production ).
I got 690-ish fps , single digit velocity spreads , and tight groups. Vel from 4in Victory Model. I want to say that I was using BULL-X 158swc's (well regarded brand at the time , used standard Magma Engeneering mold designs ) , but might have been Speer 158 SWCHP's . The Speers , and swc and rn from BULL-X can vels so close to each other , I interchanged the data between them.[FWIW, in those days CCI std primers , and always a heavy crimp.]
MrS's guns may have had differences in chamber and throat dimentions , or his bullets could have had marked difference in bearing surface and sizing. Just goes again to point out the importance of chronographs. They are inexpensive enough that all handloaders should have one , and any one venturing beyond the most common flavors of loads Really Needs one.
As much as I like WW231 , for loads of the rodent fart variety , I would use powders even better suited. Bullseye is justly legandary for reliable and consistant ignition , even with very small charges. Red Dot with its lighter density will take up more space , give a higher Load Density , which is theoretically a good thing.
IMR give a minimum recomendation of 50% Load Density with Trail Boss. Try. 50% load density (with your particular bullet) , and see what type of vels you get.
And depending on how much effort you care for shorten the cases , or use .38Long Colt (somewhat available due to CAS usage) , or .38 Short Colt (available periodically )
And see , I didn't even mention the words "case filler" , because that would open a 5gal bucket of worms , and is truely without a definative answer.