The VAST majority of what you read about “killing ability” of various rounds has absolutely nothing to do with facts or science, and everything to do with conjecture, old wives tales, and “grand pappy said”.
Bullets kill by destroying tissue. The more tissue destroyed, the faster things die.
Picture the wound channel in 3D. Three things matter in the big picture-
Placement of the wound.
Depth of the wound.
Width of the wound.
The first is controlled by you. The easier the rifle is to shoot, the better placement will be. The second and third items are the bullet construction. It is possible for a 300 grain 338cal bullet to create a wider wound at the same depth, or a deeper wound at the same width the with a 200 grain 30cal, and the same with a 200gr 30cal over a 147gr 6.5mm. It’s all bullet construction and the type of upset that each bullet differs in. I.E.- it is possible for a .224 cal bullet to create a bigger wound, and consequently kill faster than a 300 magnum depending on bullet.
Screw the magnums. You, learning to shoot at distance need well setup rifles and serious constructive round counts. The best favor you could do yourself is to get a good, fast twist 223 in a lightish rifle, good scope, cases of 77gr match ammo, and try your darndest to shoot it out before next year. Then, get a light recoiling, easy to shoot rifle (6.5 Creedmoor) set up similarly to hunt anything bigger than deer.