The life expectancy of a gun does depend on many circumstances though the main reasons are far fewer in number. I would start with the gun its self, it should be well designed with no over complications. Using the best materials available at the time of its manufacturer, with the most important thing of all taken good care of by its owner.
The only example I have that can genuinely prove my point is the oldest gun in my collection which I have used on a continuous basis is my Purdey hammer gun built in 1869.
The guns design is as simple as an early breach opening gun could be, having external hammers though it does have a rebounding lock design. Each lock has just the minimum number of working parts with no extra add ones each built by Joseph Brazier. No top leaver and its associated spring also no complicated ejector mechanism, with just a central cartridge extractor mechanism which it does whether the cartridges have been fired or not. The breach opening mechanism is via a heavily built thumb leaver set in the trigger guard with a rather large and well-proportioned spring to keep the barrel locking bolt tightly in position.
Bearing in mind that the gun with its Damascus barrels was built in the days of black powder then proofed for Nitro powder use and is still well within the British proof limits today.
As for cartridge used given the guns age and its first owner, he was a member of the British hierarchy with lots of time on his hands. He did seem spend more time on the grouse moor and the pursuit of driven game than in either houses of parliament seeing that he was Britains first Minister of Agriculture. So, a conservative estimate I feel would be somewhere in the region of 400,000 cartridges in his lifetime, as for my own efforts in cartridge use over some fifty years around 100.000 though the load weight has reduced from 1 1/8 to 7/8 ounce as I have become older. And of course, there is the capacity for the gun to do the same numbers in the future if treated like a good horse look after your horse before you look after yourself!!! The gun was put back on face in the early 1970s after I rescued it from being scraped. Mechanically all the parts are original except one pin though all the other action pin slots are badly opened up. To the disappointment of some I wont replace the pins because they are original parts of the gun and do still work correctly I see them as part of the guns character.
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