6.5x57 Swedish is a pretty weird cartridge in a good sense. Some ballistics expert will probably come on and spend 50 pages defining, extolling and explaining the "weirdness" of this cartridge but I can sum it up briefly. For some reason, this undersized, underpowered cartridge has a magically collection of physical properties that result in tremendous stopping power, accuracy, flat shooting and low recoil that is almost inexplicable. I do not own a 6.5x57 myself but I can tell you that it is one of the top cartridges ever made for ballistical properties.
30-30 winchester handloads in a single shot or double rifle are pretty impressive as well. Ballistic tip ammunition has been known to make a 30-30 shoot 5 bullets into half an inch at a 100meters. I owned such a rifle myself, a custom made falling block 30-30.
The other option would be to get that big old 400andsomething cartridge that Teddy Roosevelt used to hunt Africa. .438 ? Not positive, but definitely a good starting point. I can't honestly say I'd ever recommend or own a low power double rifle. If you need two barrels in a rifle, you probably need something .375H&H or greater in my opinion. If you don't need something that big, a single shot Ruger falling block or a EABCO Model 97 falling block is a far superior weapon to a double rifle for hard core hunting.
