Jim is correct in principle, because adequate peak pressure is what actually determines whether or not the stuff burns up. That's why powders that produce high velocities, like H110/296, only work out well in higher pressure chamberings and don't make good reduced load powders. However, I'll ague that snubbies are a special problem in that some loads that seem to work fine in 3" or 4" guns suddenly grow a lot of ME ES when moved to a 1 7/8" snubby barrel.
My surmise has been that these problem loads experience uneven ignition delays in any revolver, regardless of barrel length, due to primers unseating the bullets to an irregular degree before the powder really gets burning, and that in that circumstance the bullet base location in the barrel at the moment the peak pressure is reached has a ±tolerance that, with slower igniting powders, includes a point about 2" down the tube. I may be off base with this and some other explanation may account for the phenomenon. Regardless of the actual cause, though, going to a faster powder cleans it up a good bit.
ES: Extreme Spread
MV: Muzzle Velocity
SD: Standard Deviation
VMD: Volume Measure Density (a misnomer by itself as the units are actually the inverse of density; cc/grain or cc/gram), but more properly called VMD Factor, which clears up the unit issue, as you multiply a desired charge weight by it to get the number of cc's you would need to meter that weight if the VMD were accurate. Because the exact VMD Factor varies with each lot of powder produced, Lee instructions say the result of the calculation is an estimate.
Bulk Density: How much weight the powder has per unit volume. Since you would measure a known volume then weigh it to determine this number, it obviously includes the volume of the air spaces between the grains. If, for example, you tried to measure the density as specific gravity based on water displacement, the air spaces would fill with water an not be counted. The result you would then get is called Solid Density because that air space is removed, same as it would be if you had squeezed out all the air and pressed the powder into a solid lump.
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