Before you jump to the conclusion that .004" gap alone is what causes your 66 to need .1 gr. more HP38 to match the velosity of your 686 I suggest rereading the protion of the old Speer reloading manual titled Why Ballisticians Get Grey. In short, they tested a whole bunch of revolvers from various manufacturers and concluded that identical appearing revolvers that could have rolled off the assembly line side by side vary in velosity by 300 fps when tested with the same lot of ammo. I'll dig into by old book storage latter and edit in the Speer reloading manual number.

That said, I agree with you that flash gap size gets more attention than deserved. I think that's because it's so easy to measure.