the controversial .499LWR, which i own two of, would be my choice to full fill your curiosity, many may disagree, but the fact is, i think from over 7 years of shooting and reloading that beast, i believe with the .499LWR loaded for the intended game, it will easily kill nearly every animal on Earth, (if Ol' One- shot, Howard Hill can kill an Elephant with bow & arrow, i am quite certain a .499LWR also can, He's most famous animal is certain to be the elephant he took in 1950 while hunting and filming in Africa, gaining Howard the fame of being the first white man to kill an elephant with bow and arrow. He used a 41 inch aluminum arrow tipped with an enlarged version of his classic Howard Hill Broadhead weighing in at 1700 grains. The bow he used had a draw weight of 115 pounds.)
i say nearly, therefore quite possibly could do in a Grey Whale, if so it could be the only weapon in AR-15/M4 configuration to accomplish that task, the .499LWR case is 1/10th inch longer than the .50 Beowulf, which as you know the .357 Mag. is 1/10th inch longer than the .38 Special and you can see quite a difference there, likewise the .499LWR vs .50 Beowulf. (pssssst ! don't tell anyone, but the .499LWR, IS basically a .500 Cal. it was called a .499 to keep it from being banned in states that do NOT allow .50 Cal.shooting) the current selection of bullets available for reloading is adequate for the average reloader, i do NOT hunt, (i once did, just don't care to kill animals any more, got nothing against it) i mostly put holes in paper, tin cans plastic milk jugs filled with water etc.
soooo, there you are, my most humble OPINION about .the.499LWR.