I think once one goes past the .375, one has passed the true point of a plains game rifle. After that, it is a bigger medium bore rifle, that can be used for plains game, but recoil and power has passed the plains game rifle status.

A .375 with a 235 gr bullet or a 250 or 270 gr is a lot less than a 300 or 350 gr .416.

For a lot of people also increased recoil affects their ability to aim accurately. A person who denies this is a BSer. It is a matter of perspective and when the relative recoil makes a difference.

So it is a matter of perspective and opinion, not fact or science, but for me, the .375 is a versatile allround rifle, the .416 is a versatile medium-big bore rifle that can still be used for plains game.

PS I like to go by the old ranges, big bores sort of start at .450, with the .375 to sub-.450's being medium bores. Most call anything above a .375 a big bore.