Boeing,
Seems to me your are well set and on your way.
As far as powder differences, the biggest thing are the burn rates of powders.
Here's a great chart by Hodgdon that shows all the
major powders and their burn rates fastest to slowest.
https://www.hodgdon.com/wp-content/uploads/2017/09/burn-rate-color.pdf
Generally the fastest powders are formulated to burn fast and clean and lower pressures in large bores, ie. Shotgun loads. If you glance at the chart the top 15 or 20 fastest generally are the powders that excel in the Trap and Skeet reloading world. Field loads also. Also these loads are weighed in
Drams, not grains.
Now go to end of the chart, slowest up towards the say #150 towards #90 numbers, these are formulated to burn slowly and build pressure at a slower rate than the powders at the top of the chart. Formulated to burn/build higher pressures very slowly in larger case metallic cartridges, ie High Power Rifle and utilize the long barrel length's for clean, complete burning. Everything is based on efficiency and cleanliness, while keeping the pressure curves well below maximums for any given cartridge.
OK, that said here's a few take-aways.
Somewhere along the line, whether by design or discovery, metallic cartridge pistol reloaders found that the fastest powders when used in small and short metallic pistol cases with limited capacity could produce muzzle velocity's that were wonderful, when loaded with caution and keep pressures down.
This is why you see pistol reloaders utilizing a lot of shotgun powders.
Back in the "Old" days, I reloaded a lot of .357 Mag using
Hodgdon HS-6, which is primarily a shotgun powder.
Today, specifically speaking of the .40 S&W, my go to load is
WW231, again primarily a shotgun powder. I've used this as my "go to" for a long time for the .40S&W. Then Winchester comes out with
WSF, and some fellow hand loaders tell me to try it, it's cleaner than the WW231. Haven't made the complete switch, but I one of my minor complaints was always was
WW231 leaves guns dirty after a couple hundred rounds. Here's once again shotgun powders utilized in pistol cases.
You noted you use
HP38, it was formulated specifically for metallic pistol cases, closely mimicking the shotgun powders previously used.
Regarding your use of
HP38. I don't know if you use it in the .40S&W, however I checked some data and found that HP38 and WW231 in the .40 are identical in velocity, pressure and powder charge.
I haven't tried it, but I think volume wise HP38 occupies more case.
One point about the older, fast burning powders that were primarily designed for shotgun. Because of this, increases over maximum charges is an absolute NO-NO. Maximum means just that and the pressure curve changes DRASTICALLY after maximum. One more thing, a VERY LITTLE goes a long way. So, some powders like
BULLSEYE in a .38 SPL case and others with fast burn rates, it is possible to throw a DOUBLE charge and not know it unless you inspect the case prior to seating or (worse case) pull the trigger. Results are catastrophic destruction or missing parts and pieces of one's hand.
Powders like HP38 were developed to have the same pressures and power with more volume, making double charging a case almost impossible without detection.
So there ya go.
Sorry for my lack of brevity, but I feel so strongly about being concise on this subject. If we are a little unconcise with our trucks we just have to walk, in this subject if we are unconcise, the results can be horrible.
Anything else you have, please ask.
~Chad