The weight of the rifles is surely one of the reasons for a gun bearer. Nevertheless, you have to shoot with your double rifle and all this heavy and above all top-heavy rifles are difficult to get on the shoulder quickly and also if necessary to re-chambering cartridges quickly. If the actions are strongly built, such as the PHV-1 action of my double rifle caliber 577 Nitro Express, it is even more difficult to opening the rifle. Sure, you have two shots ready, but if you add everything up like the repeated target acquisition after the first shot and the reloading if necessary, you are much better served in all cases with a lighter double rifle,... or a bolt action rifle. However, light big bore double rifles have the disadvantage of stronger recoil with also negative consequences.
If you take into account all the advantages and disadvantages of big bore double rifles and have had some practice with the handling of such rifles in comparison with big bore bolt action rifles, one comes to the conclusion that such rifles should not exceed 12 lbs to 13 lbs in weight with a barrel length of a maximum of 26" and should have an ejector. Something like that definitely limits the choice of the cartridges and rather exclude in particular the biggest of them. This brings us back to the various cartridges between caliber 458 and 510, and in particular the two cartridges that seem very practical to me nowadays for double rifle hunters in Africa, the 470 Nitro Express or for those who want something more power, the 500 Nitro Express.