I don't have one of these but at a guess I would say it more to do with the tube and battery cap threading that is causing the voltage drop as the 510 and contacts look fine.
I don't have the Venom X either... But, if the switch is getting hot, it's the culprit of the voltage drop. The switch is actually becoming part of the load in the circuit...
It has to be providing/allowing a resistance in continuity to heat up like that... Just a guess, but more than likely, not enough surface to surface contact area creating a "resistive choke point" to the flow of current.
Some pics of the switch assembly would be helpful...
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