The holy grail of double rifles rounds is a flanged, low pressure cartridge.

The very primise of the double rifle was to deliver a large caliber bullet at moderate speed with low pressure in tropical heat. The "Low" pressure and flanged case insured no sticky spent cases to slow your reloading.

The design of the action of a double rifle does not lend it's self to high pressure rimless cases. If your desire is to have a high velocity rifle, look somewhere other than the double rifle. All double rifle will eventually "shoot of the face". A double rifle with a high pressure "magnum round" will get there a Hell of a lot sooner!

Just my opine!