What sort of price range are you looking at?

The 7x65R is a great medium cartridge for a double rifle to be used for smaller and medium sized game.

Always depends on what you want to use it for. If you want to use it also for larger plains game such as eland and the like, a larger calibre might be preferable. If you can shoot a .400 recoil should be a problem. Some cartridges to consider:
- 9.3x74R - quite mild to shoot, lots of choices of rifles to choose from. A favourite for European driven hunting. Can be used on larger game if needed in Africa where legal.
- .375 Flanged - the rimmed version of the .375 H&H Magnum. Similar to the 9.3x74R, a little more power, but legal for say buffalo in more African countries if it is needed when a shot presents itself. Might be built on a more expensive double rifle action than the 9.3mm. Something to consider.
- 8x57R - a good European cartridge, similar to a .30-06 in power.
- 8x75R - case based on the 9.3x74R in 8mm. More grunt than the 8x57R.
- 7x65R - a good cartridge in the 6.5 to 8mm range.

A rimmed cartridge is preferable to a magnum or rimless design, but the .375 H&H Mag, .30-06, etc are often also available.

Rifle and cartridge will depend on the maker and what they offer in their models. Most of the above are available. The .375 Flanged is probably the least common but more common today than in recent years. The 8x75R is less common than the others.

Some under and over designs are very accurate and can be quite affordable. Popular in Europe. Many hunters prefer the traditional side by side.

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My personal preference?

As I have a .450 NE with open sights, and would like a .577 NE, maybe one day, they will cover the big end. I am keen for a scope sighted .375 Flanged in a Heym or possibly Verney-Carron side by side. V-C makes a rimmed .375R Verney-Carron which is a "magnum" sized proprietory cartridge but may also make a .375 Flanged. Not sure and would need to check. The .375 scoped sighted DR could be used for anything a .450 could be used for - ie allow a second rifle to overlap - but also if set up right, allow lots of plains game and medium game use. Similar trajectory to a .30-06. I have a U/O 9.3x74R, it is very accurate, and a great medium game rifle. It has accounted for fourteen buffalo too so versatile.

A 7x65R DR in a light design would be nice too.

Good luck and have fun with the search and eventual rifle.