Both calibres are popular over here, with the 404 seeming to be on the up for use on big stuff at the moment.
Personally i would steer clear of a hot loads in the Rigby. A 6000 foot pound thing kicks like a mule and there is a high risk of bullet failure on heavy game at close range. Better to load the Rigby at around its original operating levels (400gr at say 2400fps) and use another, lighter weapon for longer range applications (The .338WM is a good choice for that).
That having been said there are not a lot of places over here where one would need to take shots in excess of 200m, and even that would be considered a long shot except in the desert regions.
As mentioned above, for only 2400fps and a 400gn bullet a person can use a 416Ruger. And I agree that heavier loads in a Rigby need some learning. See below.
In my opinion, the main purpose in this century for getting a 416Rigby is to use the capacity for handloads. And handloads can avoid the problem of bullet failure.
Here is a picture of a 350gn TSX shot into the face of a buffalo (70 yard) and recovered in the shoulder:
The monometal bullets do not fragment even if they lose the petals. Why in the face? I had waited several minutes for the buffalo to step out from behind some bushes. When it did it was face on.
Below is a load of 350gn TTSX in 416Rigby, getting ready for Africa. Their muzzle velocity in my 416Rigby's is 2825fps. That is like a 416Weatherby and provides for flat shooting from 0 to 400 yards.
A person needs to shoot these heavier loads with a fairly square presentation and securely inside the shoulder joint. An important point, different from lighter caliber, is to hold the forearm tightly. This is for two reasons. (1) a person wants the rifle in hand for a follow-up shot. Equally pleasing, (2) the weight of the tight forearm adds to the overall resistance to recoil and lowers the recoil. A reliable scope is also a must. I like the Nikon Inline with 5-inch eye-relief. They are the toughest scope I have found. A friend who busted about 20 Leupolds shooting thousands of heavy rounds (5000 ft-pounds and up) has not been able to bust a Nikon in several more thousand rounds. Someday one will break, no doubt. But Nikon are proven to be tough. Why a scope? For the buffalo above, and oribi below.
A flat shooting rifle is nice on little oribi-sized animals. Here is a 202-yard shot with the 416Rigby on a 35-pound oribi:
However, I agree that a person needs to learn to shoot it until it is just 'another rifle', like a 308 or 270. And when I reach my 70's I expect that I'll probably downsize to a
416Ruger, maybe shooting 330gnGSC at about 2600fps muzzle velocity. That is down to 4950 ft-pounds muzzle energy. If I were starting out, I would recommend a 416 Ruger.
While I have a 338 WM, I tend to walk the forest with one rifle at a time. If a buffalo is included in the menu, then a heavier caliber is better. A couple of Christmases ago we were looking for an impala and the 416 just had to do.
The splatter on the skin is around the entry wound.