I have been playing with shock collars and cattle prods as well.
Here my observation:
The market now is swept with very cheap products from China and I am a bit concerned with them. Often these do have a significant DC level in addition to AC. DC barely causes direct pain, but it can lead to burns (I have experienced it myself).
Also with cattle prods, I only use brand-name products such as Hot-Shot.
I have stayed away from stunning weapons, as I am more looking for a tool that can inflict significant pain, rather than the stunning-effect.

Also, I have developed and built several tools myself.
I found a voltage level of 500-1000 Volt to be ideal. Obviously with limiting the current to safe levels! Higher voltage is only needed if you want to penetrate clothing, Lower levels may provide inconsistent results depending on the dryness of the skin.
A very big variable however is the frequency. There is a very cool video here: https://www.youtube.com/watch?v=MMzU66IHe-k

The challenge when applying this research to commercially available products is that they don't produce clean sine waves. When adjusting the pulse length at a constant frequency, all one does is change the harmonics and the DC part. And often it is the higher harmonics that actually cause the significant pain.
And, unfortunately, in most cases the pulses are only positive (or negative however your interpretation). This again means there is a significant DC component. All this is information one doesn't find in the specs. It only becomes visible once putting the device on an oscilloscope.

This is why I design my own equipment. If I use any commercial product, I protect myself from DC or malfunction by running the current through a capacitor (something which I have also built into my own devices).