Agree with almost all the comments above; the biggest causes of voltage drop (in no particular order):
* Quality of battery - good quality (not fAW's) have lower internal resistance and accordingly lower voltage drop will be experienced with good quality batteries.
* Oxidizing on battery (negative as well as positive pole - not always visible, but mainly caused by arching) - solutions as mentioned in previous posts; contact cleaner, white rubber etc.
* Loose/dirty nipple that connects centre pin, firing contact and feed tube - clean threads on centre pin & nipple and tighten properly. Also make sure the thin piece of silicone on firing contact is not pinched between centre pin and nipple - you don't want any insulation but a clean contact here.
* Dirty firing contact (folded piece that makes contact with battery positive pole) - clean as suggested in previous posts, as prevention for oxidizing layer on battery positive pole, use Noalox, especially if you don't fully press your firing button which will cause arching without Noalox, even with sub-ohm kit installed.
*Excessive old juice in 510 connector - over time this old juice will become slightly conductive and cause "creeping" of current, although it's minimal, it will influence the voltage drop negatively.
* Bottom spring screw loose - inspect and make sure the spring screw is tight - If you do serious sub-ohm coils, remove the paint or anodizing where the spring makes contact with the Reo (after I did it with mine, the voltage drop improved from 0.201Ω to 0.188Ω with a 0.65Ω coil.
* Loose Atty coil posts - secure all posts and make sure atty is fully screwed down into the 510 connector (don't over tighten just secure) plus make sure the 510 threads on atty as well as connector are clean.
