Howdy, Driftwood. Been awhile...
I wish I could publish some of the pressure-vs-time curves I've recorded in both rifles and revolvers. Not sure how to do it...but yours shows the relative characteristics of the curves between BP and some smokeless. Even if the PEAK pressures come out the same, the rate of onset is important, if not critical! I HAVE gotten smokeless loads in .45-70 that were close in profile to BP, but it can be tricky and really requires the pressure measurement equipment to do it right!
There is another hazard with very LIGHT smokeless loads, especially (but NOT limited to) revolvers: That is the danger of a charge so light that the bullet becomes stuck in the FORCING CONE, so that the barrel-cylinder gap is stopped up. If a charge of smokeless is not ignited completely but the bullet leaves the mouth of the cartridge case, and lodges in the forcing cone, the pressure will increase. Since smokeless powder is progressive-burning, i.e., the higher the pressure, the faster the burning rate, which increases the pressure faster, building on itself. Normally, when a bullet leaves the case and passes down the bore, the pressure drops, slowing the burning rate. But if the bullet stops, so the pressure isn't relieved, the pressure will build up exponentially. If the pressure in the chamber of a thin cartridge case, like .45 LC and .44-40, builds up past about 35,000-40,000 psi (NOT CUP!), and the chamber is fairly sloppy (as is the situation in many .45 LC, the case may rupture. The resulting escaping gas acts like a cutting torch on the steel. The result is usually the top of the cylinder lifting off, taking the topstrap with it! This can occur in modern-made Colt clones, and even Ruger Old Model Vaqueros, though the later is much less likely to fail catastrophically.
This can occur with medium burning pistol powders that are loaded DOWN to the point where the initial pressure is BELOW 5000 psi. Ironically, this is less of a problem with some of the faster powders like Bullseye, but you can easily double charge those powders, and the small amount of powder can slop around in the case. That is why Trail Boss was developed, but it isn't a cureall, and I prefer to load that powder NO LIGHTER than the middle load shown in the loading data! It may not be rocket science...butdang near! (Take it from an old rocket enigneer!)