@OmSigDAVID,
there are over 100 type of gunpowder available to reloaders. about 1/3rd of them are suitable for loading handgun ammo. Use your local library's interlibrary book loan or e book system. There's tons, and I mean tons, of info about guns. anything told to you here is very fragmentary.
MOST, but not all modern gunpowder is much more potent (per gram) than black powder was. A 44-40 had 40 grs of black powder loaded under a 200 gr bullet. Today, that same ballistic performance is attained with 10 grs of smokeless powder. 10 grs of a very fast powder, such as Alliant Bullseye, intended for use in 380 pocket pistol load, would blow apart the 44-40 revolver.
No, +P+ .38 ammo did NOT replicate 357 full load performance, not even close, actually. It was 110 grs at 1200 fps, while the full charge 357 load is 1500 fps, both from a 4" long barrel. that "extra" 300 fps adds quite significantly to the likelihood of a man's being shot with it also being stopped instantly.