"It's simple, any experienced DG hunter can tell you, that a 500 is far more a killer and effective than a 450 and a 416 and a 375 etc." True of a 50 cal BMG also!

No argument there and I agree that for the professional hunter the bigger boomers may be best.

But what about all their clients that shoot lesser rifles and actually kill a lot of game?

Or some odd ball caliber from another country like the USA?

Here are two of mine that have taken DG --- they are old and mostly forgotten, but their bullets still kill large and dangerous game and the game neither know nor care where the bullets came from.

Winchester 1895 .405 WCF with same performance as 450/400.

Winchester 1886 .45-90 (.458 2.4) with 96% capacity of .458 Win Mag.
Shooting 450 grain bullets at 2150 fps and used by an experienced hunter took multiple buff, an ele, leopard and plains game in Africa. Plus Bison, Nilgai and many more in the USA.

Even though it is not an English DR, in the hands of a rifleman, no game is safe!

PS The rifleman that took the ele mentioned above just took two Alaskan Grizzlies with a muzzle loader and is on his way back to Texas. We will be chasing warthogs together this winter in the Rio Grande Valley.