These recommendations are made utilizing research from chuckhawks.com and Bass Pro - 1 Source (see link and image below). Also refrenced John Taylor's book African Rifles and Cartridges. John Taylor was a famous African Dangerous Game hunter that developed a formula for determining the knock out power required to bring down an Elephant. Although the formula has been disputed by many as it doesn't take into account the type and composition of the actual bullet itself, I believe it is a good reference.
This webpage leverages his formula and matches bullet speed, weight, diameter, and estimated animal TKOF. As the distance increases the velocity of the bullet decreases. The velocities of certain quality ammo has been recorded into a database, this is then referenced and compared to the requirements for specific game. Federal's website was utilized extensively to get ammunition ballistic data for each ammunition. Federal Premium Ballistics Calculator This webite does not take into account utilizing an overpowered weapon for game at a shorter distance. Shot placement is a huge factor in hunting success! Finally, the chart below from Bass Pro was also used in consideration for specific caliber and ammunition for game selected. I believe this chart was also available in the American Rifleman magazine, but I haven't been able to locate it again..
Bass Pro - 1 Source - Use This Rifle Caliber Chart to Pick the Right Ammo for HuntingThe advise given on gun selection is only a rough estimate. I also recommend you buy the best ammunition possible. My thought behind this is; it is better to do everything in your power to make a good clean kill. This is for your sake and the animals. I have found through personal experience that Barne's makes an excellent bullet. And if you don't make your own ammunition Barnes, and Buffalo Boar are excellent choices for boxed ammunition.
This website is still a work in progress, so please consult with a professional and do your research before purchasing a rifle for your hunt.