How do you pre kill a m/rat?

If you're going to bash a mouse against something, you should just flick it in the back of the head or smack it with a ruler. You have more control over the force, and the target area.

Throwing a rodent against a hard surface will not always kill it. If you have to throw twice, you shouldn't be throwing it at all. This applies to all methods. If you have to use it twice on the same animal, you shouldn't use it at all.

I'm in the process of doing the same thing, switching some live eaters over to f/t mice. There are more humane ways to do it than I do it, I know, but I don't kill them, I just knock them out. This probably wouldn't work for rats, but for mice, it works fine. I usually just give them a good flick in the back of their head and it seems to put them out. they don't struggle when the snake wraps them up, and that way you don't have to watch them twitch and what not...

If you're flicking them, try flicking a bit harder. It should kill them.

Cement floor + mouse + blunt object = dead mouse

I basically put them on the cement floor with a towel or soemthing under them to catch blood/urine. Light tension on their tail causes them to cling to the towel and not move. Then I just whack them on the head. Twitch, curl, dead. I tend to use the hand of a screwdriver. Probably overkill, but it ensures that they're dead with one blow.

It's considerably more instant than CO2.