I've ran dual 18350s with dual cartomizers for a end resistance of around 2 ohms. That's a LOT of power, and the hot spring never failed. 99% sure this is a short.
Likely what's going on here is a short somewhere in your atty/carto and/or 510 connection. Either tightening the heads down too much and squishing the rubber insulator ring on the Bolt's 510 connection, causing the positive and negative to make a connection, shorting out.
Or, if you are using some type of clearo system, those can also short out. The floating positive connection, which flows current through the center pin of the Bolt's connection to the center pin of the replaceable head moves depending on where it has room to move.
If you have the clearo unscrewed from a device, and tightly screw the top cap down it will cause the 510's center pin to stick out more. If you remove the top cap, and screw the clearo onto the device, it will cause the center pin to stick up more inside the coil head to base connection. Either way, if too much pressure is in the wrong area, it will squish the insulator and short out.
The space inside the Bolt's head is pretty cramped, it's possible a wire got pinched somewhere and shorted out against the body. The bolt uses the body as the ground, and has a single wire coming from the connection's center pin, to the switch, then from the switch to the positive connection for the battery. If one of these was touching the body, it would cause a short. It would likely have to be the top wire, the one that attaches to the 510 connector, because if it was the wire coming from the battery to the switch, it would have shorted out and fried the second the battery was connected, not only when the switch is depressed.
The switch in the bolts is crappy IMHO, but its death results in poor performance and low power, not shorting and catching fire. I'm 99% positive it's either a short in your 510 connection (bolt or heads) or something inside if the heads. The wire shorting out in the head is possible but unlikely.
