As per the title and all your experiences... What is the Furthest, Flattest Shooting Double Rifle Caliber?
It depends...
Double rifles for dangerous game
There are not too many built for distance. Clearly, as already indicated (e.g.
Green Chile,
VertigoBE,
Red Leg) the .500/.416 and .375 H&H (Flanged or not - both exist) are the best candidates.
The most common, and best candidate, in .40+ DG caliber, is the .500/.416.
The ubiquitous (in Europe) 9.3x74 is tittering on the edge of the answer, but it is a bit weak in power...
Double rifles for plains game
There are a vast number of them, generally of Continental manufacture, but also a few British, e.g.
318AE's 280 rimless NE, and some beautiful Holland & Holland in .300 H&H Flanged.
Rimed cartridges
As also already indicated (e.g.
Wishfulthinker580,
VertigoBE) 7x65R, 8x57R are "standard" calibers. You will also find a few double rifles on the German and Austrian markets, likely custom made for specific customers, in 5.6x50R, 5.6x52R, 6.5x57R, 6.5x65R, 7x57R, basically everything with a R (for rimmed), although most of the these calibers are chambered nowadays in single shot rifles (Kipplauf) and combination rifles (Bergstutzen). A few doubles have made their various ways to the US.
As also already indicated (e.g.
Wishfulthinker580,
Hunter-Habib), .300 H&H Flanged and 30R Blaser are in the "magnum" class. The 6.5x68R was also a powerhouse, but I do not think that ammo is produced for it anymore. Its place has been taken by the 6.5x65R.
The previously mentioned, and ubiquitous in Europe, 9.3x74R is again tittering on the edge of the answer, but it is a bit shy in range compared to the above.
The most common, and best candidates, in modern production are 7x65R and 30R Blaser.
Non-rimed (belted or not) cartridges
There are not a whole lot in pre-WW II rifles, due to the number of R cartridges, and the most common in modern production, initially made for the US market, although the cartridge also has a great reputation as a hunting cartridge in Germany, is the already mentioned (
Red Leg) .30-06.
I have also seen a few Browning Express Superposed and Heym double rifle in .300 Win Mag. They have excellent reputation in Europe.
Multi purpose doubles rifles - several sets of barrels
Any combination of the above, plus a lot more choice for the DG barrel set (.470 NE, .500 NE).
Blaser and Heym are the modern makers that seem to have captured most of this market, and
Red Leg's Blaser is a great example. There are occasionally on the used market Heym rifles with 3 to 5 sets of barrels................
Heym SLE 5 Bbl Set Cased for sale online.
www.gunsinternational.com
Edit: looks like Mtn_Infantry and I were typing at the same time with the same idea.