Use the flat (wc) seating stem for the swc's, I have loaded countless 1000's of 38spl's for decades using that same rcbs/wc combo.
You don't need to worry about seating and crimping in different stages if you're using a "standard" roll crimp.
3.3gr to 3.6gr of bullseye will get you to the promised land. And yes, .3gr will make a difference. It depends on the length of the bullet's body/depth seated in case and the diameter of the cylinders. You're matching the pressure/seal/expansion of the case to your clyinders.
I've cast my own swc's for decades using some of the classic molds/designs:
lyman 358429/358431/358439
lyman 358156
lyman 358477
h&g #51
Of those bullets the 150gr 358477 always out performed the other accuracy wise along with being more accurate with a wider range of loads.
Another bullet to look at in your quest for an "accurate" 38spl target load is a fn (flat nose) or rn (round nose) design that doesn't have a shoulder. They've changed the angle on the forcing cones of modern revolvers to shoot the swc's/shouldered bullets more accurately. But any mistakes in reloading (angled bullets/loaded off center) or any fouling/buildup on the forcing cone or timing issues will put the bullet's shoulder on a collision course with the forcing cone.
This fn bullet has quickly become my favorite for the 38spl/357's. The best of 3 different worlds. It's extremely accurate, lethal, 2 crimp grooves that gives the reloader the ability to move the bullet in or out into the different sized cylinders/loading for accuracy.
The 2 targets on the right are those 640's (pictured above) using 2 of my favorite 38spl target powders. Note the 3.5gr on int, that load has been extremely accurate in every 38spl/357 I've ever tried it in with any 158gr bullet. You want to see if your revolver likes a bullet/lube/seating depth, try it with 3.5gr of int (int ='s international clays). The bullseye powder I usually start out at 3.3gr and work up to 3.6gr looking for accuracy.
Why those pictures/loads don't look like much, 6-shot groups @ 50ft. I wanted a load that is accurate enough to shoot shotgun shells setup on the bowling pin table & 50ft. Want to give the guys using 22's a run for their money. That load is accurate enough to hold the x-ring on the nra 50ft target.
I'm in the process of loading 4,000 of those mihec 640's/3.5gr of int to play with.
Something else to think about, you should look into coated bullets. Put 200 rounds of those bullets pictured above down the tube in my 686. The bbl's/forcing cones stay clean along with the cylinders/ring build-up. No build-up ='s accuracy. Not much to clean-up after 200rounds.
Just something to think about seeing how you're just getting started.
