I would think it might be a little harder to aim on a small chicken head? As opposed to say a bigger mammal or something. I know I would have a hard time hitting a chicken head lol but if you have good aim then I don't think there would be a problem and it's definitely not an inhumane or slow...
Sorry but that doesn't sound humane at all and also, not to mention... it took a long time and is way more effort than just chopping his head off or any other method. Getting the barrel, putting him under it, and your elaborate method with the hose and tail pipe and sealing it and everything...