Is there anything wrong with...

[6mm Remington]

My pick would be the 45-70.

 

[Charlie-NY]

When I researched the popularity of cartridges used for mountain lion hunting the two that popped up most frequently were 22 rimfire mag and 30-30. Presumably, everyone has got a 30-30 laying around which probably lead to its use across the hunting spectrum. It's apparent that the 22 mag is sufficient for the task and probably does the least pelt damage.

Just a few thoughts on the topic.

 

[1100 Remington Man]

Rifle what ever is lightest for a longer shot but if you have the opportunity to take it with a handgun, I think you would love that memory.