I'm finding the spring is the primary problem on the negative side.
Surprisingly enough, I had the thought to put the battery in backward, and lost a good portion of the formerly positive contact voltage drop.
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That is why the new sub-ohm kit that Rob is putting out contains a new spring with significantly less voltage drop (it is made of beryllium copper, if I remember correctly). It also contains a new positive contact. There is a thread where people have modded the Reo to use a automotive fuse rather than the spring.. See this thread. Or, you could do your own thing, or wait for the mod kit to come out.