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I knew 19 shots was probably overkill, but I saw enough variability that I thought I should just do more to see what was happening.   What I have noticed is that it seems difficult to actually "see" when to pull the trigger.  I almost always come away with a lingering thought of "was that really it?"  I seem to remember that there is something like that when pulling the trigger with a target rifle.  Wonder if it has something to do with the blind spot where the optic nerve comes in. 

That's an interesting formula for the effect of multiple shots.  I am a firm believer now in multiple, but not too many, shots to arrive at an observation number.  

Again, thanks for taking the time to reply. 

Jim Rives