Cartridge Length = amount of 'drop' when the action opens. In a SXS, the action only needs to open as far as necessary to bring the chambers up above the top of the action. Cartridge length is not an issue as once it passes that imaginary line, there is an infinite amount of room to slide in the longest cases, for example, a 475 No. 2 at more than 4".

On an O/U, the longer the intended cartridge, the farther the action must open to provide ready access to the lower barrel. In order to get say a 475 No. 2 into an O/U, one would need to have the action open almost 90 deg.

There is a relatively small distance between the breeches and the chamber of any O/U based on the inherent mechanics of the action. Opening too wide makes for awkward actions, slow handling, and potential stress on the action itself. Indeed, given less room to 'maneuver' an O/U is a tad slower to break open and reload than a SXS. This is not a problem with most chamberings, but the larger and longer the round, the more time and care needed to reload.

The limit for most O/U is usually 9.3X74R or a 375 H&H such as in my Merkel 323E. The ones you see in 458 are pushing the limits, and one should also recall that the 458 is a fairly compact cartridge. The longer NE rounds or bolt action big wigs such as the 416 Rigby are generally too long and too bulky to function smoothly in an O/E. Dave