Marty -
Fact is, smokeless powder needs high pressure for a complete (clean) burn.
When the gun fires, the powder starts to burn, the pressure increases; the bullet leaves the case, the combustion chamber volume increases. Continued burning keeps the pressure up, but expanding combustion chamber volume keeps the pressure from rising too much.
If you want lighter loads and just decrease the powder charge to get there, then the expanding combustion chamber volume is likely to prevail. You get incomplete combustion and a dirty burn.
Instead of decreasing the charge weight, select a faster burning powder. That will get the pressure up and get the burn done in a shorter time, while combustion chamber volume is still small.
Conversely, if you want to increase the muzzle velocity do not try to get there by just dumping in more powder. The pressure will come up too fast relative to the rate of combustion chamber expansion, and you are in danger of an overpressure and blowing up yr gun. Instead, select a slower burning powder.