Yeah co2. And not just extras. All my snakes will take f/t so I kill large groups when they're the right size, that way I don't have to worry about having the right size next feeding day.
I also use cervical dislocation if I'm just p/k-ing one or two in cases where for some reason I don't have one in the freezer.
And my boa and my king hunt live really efficiently so they occasionally eat that way.