Middle power rifle with a sub 3000 FPS Bullet.
7x57 Mauser (standard Ruger m77) is your best choice.
Really a .30-30 is a pretty optimum Cougar Rifle.
Personally I would use a Marlin .44 Mag carbine because it weighs 5 pounds but also has a powerful but not very high velocity bullet.
Seen too many very badly damaged animals from Magnums up close.
223 might work but you have a problem of a REALLY light big game rifle with a 3200 PPS Slug up close.