I totally agree with Bigcountry if the animal is aware that you are there and maybe a threat the adrenaline starts pumping and the knocking them right down may or may not happen no matter the size of the caliber. I like the 54. cal under 100yds to take the wind out of their sails right away. Any big and slow bullet at closer ranges will for sure make a bigger difference on knockdown's.

I think the first thing a person should confront is their ranges that they will be using the caliber for. If you never going to take a shot past 150 to 200yds then I wouldn't think you would ever need a fast caliber. I would opt for big and slow like a muzzleloaders, lever guns like the 45-70 or 444 marlin, and under a 100yds the 12ga slug is one chunk of lead for knocking things flat.

300yd limitation the 338win mag would be an excellent choice.

400 to 500yd limitation the faster flatter mid range calibers like the 300 win mag will certainly knock them flat if your praticed enough to make a good hit. I don't recommend guys to purchase a ultra mag class of caliber unless they are familiar with intense recoil or are will to put up with the noise of recoil reducers.