I don't know anything about lions, but the San Fransisco Police Dept has found 25-30 .40 S&W's will eventually stop a 300 lb tiger.
From:
http://www.nytimes.com/2007/12/28/us/28tiger.html?ref=us
"The tiger was killed by three shots fired by four police officers."
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Regarding the use of a 270--Don't think of it as lion hunting, think of it as close quarter combat with a lion. Still think a 270 is OK?
A 270 might be OK if you can be a lion sniper.
Bruce