Northern Shooter! Thank you for your insightful post. You articulated what I have been wanting to say here for quite some time, but never got around to doing. When one lives long enough one gets the benefit of perspective. Perhaps no closer to having a solution or answer but definitely perspective!
I was fortunate enough to go to Africa in the summer of 1989 when I had just turned 20 years old. Back then a double rifle in a large bore (or in any caliber) was the stuff of films and old safari books. In the field, in the then South Africa anyway, was the "Brno 602" in either .375 H&H or in .458 WinMag. That was pretty much it. Visiting foreign Hunters would have the same calibers usually in a Winchester M70, Ruger Mark II, or Remington 700.
Then in circa 1990 the firearms/ammo industry had a safari renaissance of sorts. Federal came back with loading for the .470 Nitro Express and several gunmakers released/re-released the .416 calibers (.416 Rigby, .416 Remington, .416 Weatherby etc) with ammo makers following suit. Ruger itself resurrected the venerable .416 Riby in both it's Mark II and No. 1 models. CZ did the same and this once great caliber, which had been relegated to the safari history books, was back for another 30 year run. Ditto for the .375 H&H cartridge which had been chambered by almost every major gun manufacture in the world as a standard offering. So much so as to be taken for granted as a caliber (and ammo) one could always find worldwide with ease.
This renaissance extended to the aforementioned rare double rifle cartridges which had almost disappeared in the 1960's with the twin disappearance of Kynoch ammo along with the British Empire. Not only did Kynoch return from the dead but many hitherto rare calibers such as .375 Flanged were back being offered by Norma, Kynoch, and Hornady.
Now history is repeating itself. First the .375 Flanged ammo disappeared. Kynoch is once again defunct and both Norma and Hornady don'f offer it. If you're one who invested in a double by Heym, Chapuis, or Krieghoff in this cartridge you're finding yourself where Indian Maharaja's and European owners were in the 1960's. Ditto now for the 450/400 NE 3" which was until recently one of the most popular offerings for a double (or single shot) rifle. Only Hornady is loading for this caliber and only occasionally so. Those who invested in expensive rifles for this caliber can't be feeling too comfortable these days!
But the real concern here is the slow disappearance of the former mainstream safari calibers that were for the everyman. Specifically the .375 H&H. As NS pointed out, there are no mainstream firearms manufactures who chamber this round. Ammo makers are following suit. One used to find ammo offered in .375 H&H easily by Federal, Norma, Winchester, Remington, Kynoch, Barnes, Hornady, and Prvi Partizan just to name a few. You could purchase a box almost anywhere and in many cases .375 H&H was as easy as finding 30-06 ammo. Likewise, one could pick up a decently priced .375 H&H bolt action made by Sako, CZ, Ruger, Winchester, Remington, Dakota, Steyr, etc. Now? As Northern Shooter pointed out that list has dwindled. I believe that Blaser is one of the few left that regularly offer the .375 H&H. And the Blaser R8 is not considered an everyman-priced rifle.
As a Life Member of both SCI and DSC, this topic is something I have always wanted to raise with the influencers in the safari industry. From outfitters, firearms manufacturers, ammo makers and anyone else who has a vested interest in seeing our sport and passion live on. We as a community need to shake some sense back into the industry we support and get them to focus. Chamberings like the .375 H&H should be continued and invested in and resources not wasted on nonsense like the Ruger .375 (or Ruger .416 for that matter) with only Hornady making ammo for it. Ruger should once again chamber its rifles in .375 H&H and get Hornady to make the standard loads for it (270 and 300 grain). That would be a good first step. We can't do anything about CZ which sees no future chambering for safari calibers sadly. That means the once resurrected legacy calibers such as the .505 Gibbs, .404 Jeffrey, and even the .416 Rigby (which Ruger used to also offer) will once again only be found in the safari history books. But the .375 H&H and possibly the .458 WinMag can still be saved if our gun/ammo industry would focus and cooperate. The everyman, which the safari industry needs to survive, will appreciate it.
Sorry for the rant and apologies if any of my facts stated above are off. This is was a stream of consciousness entry which I have wanted to get off my chest for some time. I welcome anyone's comments and thoughts.
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for our kindness and generosity of offering to look after peoples rifles etc here 
