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When did the last 12 pound Napoleon guns go out of US service?
The regular Army?
The National Guard?
The State Militia?
Long Term deep storage in a warehouse somewhere?
When was the last ones sent off for scrap or museums or to be mounted in front of the public library?
The last possible moment that one could be called up for service in the Martian Invasion, Japanese landing on the West Coast, or whatever cataclysmic disaster?
I am working on a project for a War of the Worlds game, circa 1898 to 1918.
I know armies often keep obsolete items in storage for long periods of time. I know they also sometimes upgrade old weapons. Between WWI and WWII the US Army gave most of their artillery new brakes and pneumatic tires, or at least were working on it when the war came, it was a multi-year project that did not get finished before the war started.
Trying to figure out of American Civil War artillery might have been around to be fielded after 40 or 50 years. I know the US Army has 50 year old gear in storage now.
Mike Bunkermeister Creek
Bunker Talk blog
While this is not an answer to your query - I do recall in the book HG writes a scene where the RHA take on the Tripods - is quite colourful - may at least help with a scenario for your project. I am also working on a project where the US invades Western Canada in the 1870s.
It appears the 12-pounders (M1857) went out of service with the regular army at the end of the Civil War and the three inch ordinance rifle served as the main field piece until the 1880s with some modifications. Wikipedia provided that basic information - the figures on the number manufactured during the war surprised me, 1157 by Union manufacturers and 501 from Confederate sources.
Certainly some of the state militias could have received surplus pieces although you might have to research the individual state formations to find that information. Annual reports of State Adjutant Generals might be a place to start that search and continue on for information into the era of the National Guard formations. It may be that the militias of the former Confederate states were less well equipped, at least until the end of Reconstruction.
Hope that helps a little.