I have a friend in mountain lion country. He says even a pistol is of little use. You only know they are there when they are on you.
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Your friend real does not know anything about MT. lions or firearms.
Research Mt. lion attacks and you well find that large percentage of victims see their attackers before they are attacked.
Handguns are routinely used to kill and stop much larger animals.
They are 97% effective against bears MT. lions are a lot smaller.
Study of handguns and bears.
https://gunwatch.blogspot.com/2018/02/defense-against-bears-with-pistols-97_25.html