FWIW, in Terry Wieland's book Dangerous Game rifles he states the preferred lion caliber is .416 based on his and other PH's experience. Having no experience myself with lions, I suppose I would defer to his opinion. Having said that probably either the .416 Ruger or the .375 Ruger would work well and my choice of either if I wanted a new gun would probably come down to what I already had in the safe.
