The classic double calibers have been mentioned-
But the correct answer is 375 H&H! No reason you can’t get a double rifle in this super available chambering! It will also work on everything Africa if you want it to. And there will always be factory support of many different loads.
Respectfully (I’m not attacking you, just your opinion), this is the single worst bit of advice I could possibly design for the topic at hand.
375HH is the most bastardized, hot rodded, altered safari cartridge in history. From the circa 1910-1911 original load with stranded cordite (extinct), to the 1926 ICI/Kynoch load data, to its modern reproduction using IMR3031, things have changed tremendously. Since it isn’t a double rifle cartridge (It’s for magazine rifles), the factories give ZERO F’s about changing its load properties all the time while maintaining pressure and velocity from a test barrel, that means the regulation from year to year can change wildly as they swap powders at same externally observable performances.
If you had said 375 Flanged, while still a problem custom loading, it would be a far, far less severe problem.
375HH has had its velocity changed (because it’s a magazine rifle round and it can) numerous times. From the original 2400fps to the present 2650fps, running through the 235gr softs, to the 250s, 260s, 270s, and the modern 300s and super-modern 350s, it is a proverbial nightmare to handload for a double rifle due to the arbitrary variables that do not exist in a traditional Nitro Expressed rim caliber that honors the Kynoch/ICI specs, approximately.
Add to that, many newer 375HH are regulated with copper ammo like Barnes. This is a real problem in double rifles, because they are volumetrically bigger bullets, but they weigh 78% as much. That means if you try lead, the equal weight bullets don’t regulate the same, and if you go with a 300gr lead to give you the same dimensions as a 270gr copper, you cannot safely achieve regulating velocity without over-pressure issues.
Don’t do it, don’t do it, don’t do it. It’s the hardest cartridge to regulate in a double rifle and the least likely to ”just work” if you buy 30 different brands and models of factory ammo hoping for a lucky win.
