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Let me emphasize that last sentence of Dutchboy's...
If you change anything, back off a generous amount before you shoot again. That's TRIPLY important with a load that may already be past a safe maximum, as yours might be.
With smokeless powder, though, it's not a matter of the extra powder not burning that causes a loss of velocity.
What happens is that the pressure rises SO high that the bullet obturates more and more with each charge increase. As it fits tighter and tighter in the bore, the force it takes to move it goes up almost exponentially. So the velocity drops off.
But the pressure SOARS!
Using a chronograph is a safety measure when working up charges. The moment you see a dropoff or even a smaller increase in velocity, you are either at or past maximum for that gun and load combo.
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