78Kitty,
To answer your question about reduced loads, the fast single-base powders really have no lower limit. You should be able to shoot Unique, Bullseye, 700X, Clays, Universal Clays, N310, etcetera, all without any concern about a minimum.
Two situations arise that can cause problems. The first is the warning not to underload either Winchester 296 or Hodgdon H110 below limits given in the factory loading manuals. A Hodgdon technician assured me these powders are the same, made in the same plant at the same time, and put in differently labeled drums. These limits are not exact, because the Winchester and Hodgdon manuals don’t agree precisely on what they should be. But they are close. The reason is described in a warning on the Hodgdon site. They want the case fairly full because poor initial ignition can occur if the powder falls forward in the case (or if you don’t crimp adequately). Though poorly ignited, it can still push a bullet onto the rifling. In a single shot gun pressure would still build and all would be well, but in a revolver this condition leaves the barrel-cylinder gap uncovered so the powder is now unable to build pressure and fizzles away its gas. No horrible matter until you try to fire the next round into that stuck bullet. Now it can blow up.
The more commonly known problem with underloading is detonation of small charges of slow powders in large cases. Normally powder burns by combustion, the process wherein the rate of energy release is determined by the speed of the flame front, both through the mass of grains and into the individual grains from the surface. Detonation is when the energy release rate is determined by the speed of a triggering shock wave traveling through the powder mass. This condition is dangerous because the shockwave is not only faster than a flame front, it moves through the middle of the powder as well as the surface, releasing all stored energy from the powder as it passes. This happens so fast the pressure doesn’t even have time to distribute evenly through the case, and the gun is burst by very high localized pressures.
I have heard different theories for why detonation occurs, and why it occurs only with small amounts of slow powders in large cases. It is not very repeatable, so it is difficult to study. One suggests the ignition temperature is so low in the slow powder in a space too large to build pressure fast that some of the nitrocellulose melts and drives of the nitroglycerine used as a retardant. The warm nitro condenses on a cool part of the case or the bullet base, and because it is at critical temperature, when the mass gets large enough it goes off spontaneously, acting like a micro blasting cap, creating the detonating shock wave for the rest of the powder mass. If that is correct, this problem would only arise in slow double-base powders. I don’t know that it is true, so I would avoid small charges of slow single-base powders, too.
In any event, don’t worry about the fast pistol powders. Load them as small as you please.
Nick
Click on the second image here to see what leaks from a .44 Mag barrel-cylinder gap!