You could screw suspect topper on top cap. Tighten firing pin into top cap. Take it to radio shack and let them verify near 1.8 ohm. Then buy DMM and keep checking for short.
You've got an intermittent short for sure.
I suggest you do not use the mod until you have a DMM and you've figured out where the short is coming from. As has been said, short it enough times and your battery will eventually vent and that's nasty and dangerous.
Details are not abundant in this case. I'll take a wild stab at issue. Combo of switch short and topper short. On the switch end, vent hole burrs or improper switch firing pin adjustment may be causing a random auto fire condition; but this will not melt insulation w/o another issue. One suspect is intermittent topper short depending on how tight upper firing pin is adjusted.
Could be straight forward to verify issue(s) w/ a DMM.
I also think it may be two problems emus. It will be interesting to see what it really turns out to be.