No such thing as the perfect caliber since many different calibers will work fine. My favorite is the .338 win Mag. I shoot 225Gr. Nosler Partitions and they just plain work. I've shot everything from Jackels to Eland with mine. Howevr, my last Eland was shot with a 150 Gr. Partition from my .270 Win. It went down just as fast as the ones shot with the .338. Shot placement is the most important factor.