The question is rather difficult to truly answer. I think first of all the gun has to have a head start in quality of manufacture plus be of a design that lends its self to reliability and above all regular maintenance. So that does narrow things down a lot, I will put forward two guns one that has "walked the walk and talked the talk" and another that I feel with a little luck on its side will do the same, though each of the guns are a world apart in every way.
This gun has all the right things in its favor, firstly it is a true side lock with digitally operated hammers so no complicated cocking leavers and other add on parts to cause problems. It is built from the finest materials available the 1860's could offer and built by a top maker, but above all it has been carefully looked after and serviced though there was one bad patch in its life that nearly put pay to it pertinently. The gun has been in continuous use give or take the time for a restoration, plus there is enough metal left in the bores British Proof to do at least another two life times of the lucky owners without the need for a re proof. It is hard to calculate the number of cartridges it has been fed over its lifetime but I think that it must be pushing close to the million.
The other is a 1050's Baikal, The build quality is far from the best though the material quality especially the barrel steel is of the highest quality. It is a Box Lock with twin trigger so no complications there in fact very simple design altogether, one small down side coil main springs though showing no signs of weakening so far. If this gun is given the same attention of care as the first gun there is no good reason why it should not live and work and have the same cartridge count as the first, well it has done the first fifty years with no problems so far.
