Since you are not saving the meat, why not 5 rounds live ammo, wouldn't that be more humane?
I am a firm believer in the death penalty, but get a little cringey when they film the episodes of 'execution gone wrong'...rictus, body spasms, burning flesh and hair etc. when 'old sparky' takes a while...or more than one try to get it done. If prison employees are reluctant to volunteer, I would think the victim's surviving family members, if qualified, could provide volunteers.
frankly the culprits are almost never humane and frankly I don't care how long it takes. Crucified would be just fine with me.
Or as others have said. eye for an eye. Whatever you did is how you get it.