the last pics shows the action very well. its simple if you have a big lathe.

who and how to made the invisible rifling? and the canon needs a visit in the proof house.

also homemade canons only use for salut firing need a proof and if they find some kind of rifling on a breechloader chance is very high the gun will not be acceptet by the proof house. muzzle loader canons with rifled barrel are no problem.
I known a home made 76 mm canon in the napoleonic style that was made from a cut off russian T 34 tank canon. the owner was a foreman in a railway repair hall. when the russians leave germany in the early 1990s they load everything on trains. once there was such a loaded train standing over night next to the reapair area of this man to leave next morning and he had night shift. with all the stuff there was also a T 34 tank on the train thats stands as a ww2 memorial for centurys in a barrack. because darkness is your friend he take a blowtorch and the train leave the railway area next morning with a castrated T 34.
anything else was welded in working time and the canon successful got the proof.