In the past I've used a .22 LR rifle to the forehead (HV RN, didn't look for the bullets), 44 mag 270 Ranch Dog ACCOWW over a mild dose of Red Dot to the side of the head (full penetration) and 44 mag 200 Lee ACCOWW over a mild dose of Red Dot behind the ear angling forward (recovered inside skull). All worked just fine with no problems, but I figure as long as I am at it I might as well try something new. All are certainly overkill, but for the sake of research...
Options include:
44 mag .432 270 Ranch Dog ACCOWW over a stiff load of MP-300 out of a rifle or 6" revolver
357 mag .358 Lee SWC w/gas check ACCOWW over a mid range load of Herco or MP-300 out of a 6" revolver
223 FMJ out of a 16" barrel
My only concern is keeping fracturing to a minimum as the chickens will get the head to pick at after I get what I want off of it.