Most slaughter houses use bolt guns.

It's hard to find official statistics, but most definitely not.

It is clear, however, that in the United States today, “CO2 stunning of pigs is the major method that is used in large slaughter plants.” According to unpublished data from the Pig Improvement Company, the use of CO2 gas to stun pigs has increased dramatically in recent decades. In 1999, CO2 was used to stun 2 percent of all pigs and 2.2 percent of pigs in establishments that slaughtered more than 4,500 pigs per day. By 2020, those numbers had risen to 86.2 percent and 96.2 percent, respectively. Today, according to FSIS enforcement records, at least 32 slaughter plants use CO2 gas slaughter systems.

https://www.fsis.usda.gov/sites/default/files/media_file/documents/23-05-AWI-05162023.pdf

Zero pain, or as minimal amount of pain as possible. Like, microseconds.

If you assume, that the operator doesn't make any mistakes and is always 100% on point. Which they are not, as has been documented by countless of hidden cameras people have put up in slaughterhouses.

And even then, you are ignoring the immense pain and stress the animals experience the rest of their life before getting killed.