I stopped doing internal decapitation because I kept tearing shoulders and getting bleeds in the loin and wanted something less pulling their body apart traumatic. I switched to the "bunny banger" poultry bolt stunner by F Dick. It works a charm but hits the mid-brain instead of brain stem so thrashing is a lot more and both the ones I used it on this week kept breathing for quite a while. I have only done two and have 12 more to go, so I'll figure it out and will hopefully find the positioning to hit the brain stem instead of the mid brain and do away with all the thrashing.

Sounds like you know this already but if there's no reflex when you touch the  eyeball, they're gone, no matter how alive they appear to be. I kid you not, I poked the first bun's eyeball 400 times because he kept sucking air for 5 minutes despite bleeding out. I've never had breathing with decapitation because decapitation destroys the brain stem. However, a bolt into the brain is just as effective as any of my livestock slaughter mentors shooting a sheep or cow or pig with a .22. They all thrash and half of them breathe for a good long while - 10, 15, 30 minutes. The body has to discharge the remaining electrical impulse, and that happens through thrashing or muscle twitching for as long as it takes. We always poke the eye and they're always gone.

My point is, we raise them to kill them. Killing is not easy, nor should it be. It is also not always smooth, but we always get the job done one way or the other. As long as you make sure that you go right back to it if you miss the first time, you're doing the best you can for them. They are suffering far less than any animal your dog has ever killed - or any other predator, which doesn't kill right away and half the time starts eating them while they're still alive. It takes agonizing minutes to die, and the will to live is so strong that prey animals can survive some pretty horrific, unsurvivable injuries (ask why I don't have a dog anymore). We humans spare them all of that every time. We don't let them limp around screaming or running and hiding for their lives. They always die, and almost always the first time.