I'm sorry about your situation. My cat can be an asshole and has mauled and left plenty birds barely alive to die. As hard as it was for me, I've put them out of their misery with a shovel. Probably one of the hardest things I've had to do in the past.
I doubt that an airsoft gun, even a CO2 revolver would have enough power to kill something that big - even at point blank. If I were in your position I would probably use a shovel again. Whatever you use, just be sure that it could kill it and not make it suffer more.
Last edited by -Shade; October 19th, 2014 at 20:25..