I have tried three methods: broomsticking (cervical dislocation), bopping, and pellet gun. All can work; I have my own personal peculiarities which make some work better for me than others. I've detailed what I discovered when the methods were new to me to help avoid repeats of the lesons I learned.
Broomsticking works best for me with a sturdy but thin hardwood dowel, rebar, etc. Too large is hard to fit on the rabbit & crushes their neck instead of being a hard edge to sever the spinal column. I place the rabbit down, pet, stroke, talk to it, place the dowel on the back of its neck, LIGHTLY hold the bar in place with my toes (just enough to keep the rabbit from slipping out but not enough to choke it & cause bulging), then I scoop down, grab its feet and simultaneously step down while I pull up and back. Quick, when it is coordinated. (Practice with a stuffed animal or large sock tied full of rice if you struggle with coordination &want to work on the rhythm of it . Odd, I know, but you will feel a bit better prepared when the time comes.) One of the Rabbit Wringer-like devices would take away the need for all the coordination I require to broomstick. (I have not the grace of a panther, rather a tightly controlled physicality, unfortunately.) The wringers are attached to a wall so no need for reaching down to grab feet while balancing on your heels, etc. Costs a bit but worth saving for. When I do this correctly it is very quick and the rabbit goes limp. I discovered I do have strong hands so am able to do smaller growouts by hand if necessary, though the tactile experience is difficult for me (although not for the bun.)
Bopping (using a heavy stick, short bat, or other device) causes instant death when done correctly. I am not very coordinated in fine motor skills and had a bad bopping experience so I am nervous about it. It is an emergency and/or backup method for me. If you have good hand/eye, it's a good choice. You can even mark the spot where you want to hit with marker. The head is a mess after but it is a definite quick and humane end when done right. The bun will not know what's happening. If you do this, COMMIT. Set your mind to "through the target spot, not on the target spot" so it is quick and final. (That might be why I had crushed skulls, but the alternate was very traumatic for all parties involved.) No hesitating or second-guessing or underestimating force because that's where you can have problems, and no bunny deserves that. No cost for this.
Pellet gun: when I first tried this, my placement was not right, possibly underpowered gun, possibly not pumped enough, and possibly my meat mutts heads are shaped a little differently because the placement Maggie effectively uses didn't work for me. I now have the pellets with a point instead of rounded bb,, pump the gun until I can't anymore (might be bad for the gun?, but gives me the force) and I draw an imaginary "x" from the ears to the eyes & aim for the center of the "x". Actually marked in chalk on the buns head first few times when I tried this method again after a flare-up of carpal tunnel that prevented cervical dislocation. It has resulted in instant "lights out" in the eyes but a lot of nerve-release kicking from the feet. I now put the buns in a cardboard box before shooting to contain blood leakage as well as keep death throes in one area. Initial high cost for the gun & pellets, annd I haven't purchases a lead-free alterative yet so nosending the heads to the animal sanctuary. Practice on an orange or melon if you don't handle guns so you learn how much pressure to keep on it. I 've been using this method as of late.
I always have back up methods with me: a rolling pin or small bat as well as a sharp knife if all else rapidly fails. I almost always cut their throat/remove head as soon as possible after doing my preferred method , even if I'm not intending to fully process. I'm paranoid I'll paralyze, incapacitate or knock out a rabbit without realizing it so that extra step helps me sleep better at night. (Especially important to do if you're processing.)
Thank you for thinking of a plan ahead of time, and thanks to TF3 for doing the right thing when a bun was suffering, even with the challenge of having to manage it with a traumatized child in the house. I think many would have tried to avoid it and that would have been horrible for the bun, ultimately. Ah, I see I talked so long that I may be reating what others said, but will leave this as is.
