With HP-38 & W231 being the same these days (it hasn't always been true) I use 231 for .32 ACP, 32-20, .380, 9mm, 38 & 357, 357 Max, 10mm, .44 Mag and 45 ACP practice loads. 231 will seldom if ever give you max velocity loads except perhaps in the .32 & .380's. Pressure goes up quickly at top loads and if I remember correctly there are no +P loads with 231. For low to mid range loads however, it's an excellent powder, certainly cleaner than Bullseye or Unique. I've loaded .38 Special using 148 wadcutters down to minimum loads with good success and even loaded .38 Special minimum loads in .357 Mag cases for over 30 years without any problems. The same goes for using .44 Special minimum loads with 190 wadcutters in .44 mag cases. The difference in case volume between the specials and mag cases simply isn't enough to make a difference!
HP-38/W231 is a great powder and is extremely versitile for reloaders and can go from the mildest mouse fart loads to high (but not too high!) loads. I can go up to 11.0 grains with a lead or coated 240 gr. SWC and get a satisfying boom of the .44 Mag or load the 190 gr wadcutter that go wufft and are quite accurate! The stuff is hard to beat unless you want a roar and huge fireball to impress your friends and/or newbies... then you need 296/H110! For max velocity stuff it's hard to beat!
Cheers,
crkckr