There is a company in Italy building guns using known handles on 12 mm inner barrels. Generally Italian guns have been either 11 mm or 13 mm, the latter being the original standard bore size for many decades, basically equivalent to 1/2”. In Russia and Ukraine this 12 mm size is more common as their guns tend to be smaller for low visibility shooting, the Russian “Taimen” having both 10 mm and 11 mm dimeter inner barrels, while “Pelengas” uses 12 mm. When you use stainless steel inner barrels you can risk occasional barrel touch as the spears bend under strong propulsion inside the guns. Conventional wisdom used to be approximately a 2 mm gap around the shaft, remembering that the shaft tail stop is usually an extra millimetre in diameter than the spear diameter.
More on them here. https://www.pescasublog.it/tag/calibro12/
More on them here. https://www.pescasublog.it/tag/calibro12/
