The spear velocity tests conducted by Sergiy Kravchenko some years ago used a "Black Sea" hydropneumatic speargun as the spears of different diameters could be loaded with minimal effort, the charging of the gun being accomplished with the working of the rear mounted hydropump that injected the water that would later drive the spear from the gun. An internal by-pass valve, the "releasing" or "firing" valve in conjunction with the "trigger valve" being open, allowed water injected by muzzle insertion of the spear to pass right through and exit the gun without incurring any loading effort. The only common factor was the diameter of the "charge piston" on the spear tails that carried the sealing "O" ring for the inner barrel bore. This "charge piston" is also the line slide stop and sits in the "drag shadow" of the line slide as the spear flies to the target. A hydropneumatic speargun with a 9 mm bore needs a clearance of about 1 mm for the shaft, so shafts of 8, 7 and 6 mm could be fired from the same gun (or any sizes in-between if they were available). Although I have not heard of much smaller shaft diameters being used it is possible that even smaller shafts could be fired from such a gun, but smaller diameter shafts may whip inside the bore of the inner barrel given enough side clearance to the barrel wall. A version of the gun was available with a 10 mm bore, hence 9 mm shafts could be propelled from that gun, but at the expense of considerable recoil unless the gun was ballasted for such shots.
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