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thanks, had not thought the big bore maxim was available so quickly

don't forget the hotchkiss revolver canon beside of seeing limited use in the great war was dismantled first to build the trench canons

a friend was looking for an idea to build a breech loading canon, they are legal here with a smooth barrel.
For an easy build I recommend the trench canon minimized a little bit for a 25 mm caliber barrel and 20x128 cases( fired for free from the army) and cut down on the shoulder to give a 100 mm long rimless case whats is no problem because of the shell holder breechblock.

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This sounds like a wild project Lancaster!
What would the 'action' be.....one of these old suckers?
I think you'd need a group of blokes to lift it if they could, if not a crane!
As to rifling, perhaps some 'invisible rifling' might help on the smooth bore side of things?
Being cheeky there I know!