As a Butcher years ago, I've been to slaughter houses and spoken with lots of USDA inspectors.
99.9% of the time, the stress the cow feels is usually from the cattle
fences and enclosed area they herd them into prior to the kill. Not the kill
They walk into a closed fenced area, are clamped tight by a fence,
a 8" nail like spike is pnumatically driven into their skull/brain and
they drop. Dead. No flailing or screaming etc..
There are exceptions and usually is due to the person manning the nail gun missing the mark on the head. Bulls have been knocked out by the pin,
dropped into the Kill floor below thinking that they were dead, woke up when they hit and tore the place apart..