One good way to check the quality of your stun gun

Most stun guns have an internal "spark gap". This is basically two strips of metal placed close enough together on a circuit board for high voltage to arc across. It is a necessary component of the stun gun.

Unfortunately this spark gap design has a major technical flaw which causes it to fail frequently and also have a limited overall useful life. Internal spark gaps are susceptible to humidity and moisture and frequently corrode or pit, thereby causing the entire circuit and device to fail. In addition, the use of the internal spark gap limits the power of the electrical shock, thereby reducing the effectiveness of the entire device.

Luckily modern technology provided a solution...

After problems were found with the original spark gap stun gun design, inventors soon came up with a solution.  It was found that the conventional internal spark gap could be replaced with a pair of surge arrestors, eliminating pitting and corrosion problems and extending the lifetime of stun guns.

A light-duty surge arrestor

Remember when cars not too long ago used "flashers" to control the turn signals?  This was basically a relay switch which cycled on and off mechanically.  They work well at first, but any circuit component which has moving parts is at a risk of failure.  Just like we entered the age of "solid state" when transistors replaced vacuum tubes, these new surge arrestors replace the old fashioned and unreliable spark gap.