Perusing another thread on roller/dingo mods where atmizoo mentioned how the male 510 connectors varies significantly on different heads. Everyone's focused on the tail end spring, but a sloppy fitting head will have as much affect on voltage drop as the spring and switch. May be, just guessing here so bear with me, the different reported voltage drops. To test the true voltage drop of the mod, seems a 1 ohm resister between the 510 contact and dmm lead (other dmm lead on the other side of the 510 connector) would be a better & more consistent method than testing across the coil on different heads, and eliminate any atomizer fit, coil, or plating differences.