300 win mag with a muzzle break and a good trigger.
Since its 30 cal it punches a big hole, it shoots flat and has big time knock down. A 165 grain 300wm bullet has the same energy @ 400 yards as a 270, 130 grain bullet @ 300 yards and the two have almost the same point of impact.A 165 gr 300wm has more energy @ 300 yards than a 165 308 @ 150 yards. Of course all these numbers are with off the shelf ammo.
The only drawback to this setup, you have to wear hearing protection when you shoot, unless you want it to feel like someone stuck an ice pick in your ear.