The spear tails on the "Taimen" speargun shafts have a small diameter end projection which fits into the matching hole in the face of the piston, but unlike most Italian pneumatic guns this projection does not appear to be tapered, instead being just a short cylinder with a rounded tip at the rear. From my reading of some Russian forum discussions it seems that the spears are held in the gun only by the vacuum created when pushing the spear into the gun. That means that atmospheric pressure (or ambient pressure at depth which will be greater) has to act over the cross-sectional area of the spear shaft and apply sufficient force at the muzzle opening to stop the shaft falling out of the gun with the muzzle pointed downwards. Assuming that the pressure in the charged up to shoot gun's inner barrel is zero, or very close to it, and calculating the shaft cross-sectional areas for the various spear diameters used which are 6.5 mm, 7.0 mm and 8.0 mm, we obtain areas of 33.18 sq. mm, 38.48 sq. mm and 50.26 sq. mm. Thus the maximum shaft weight, including accessories, that can be supported is 343 g., 398 g. and 519 g. respectively. For a 5/16 inch diameter shaft the figure would be 511 g., providing a comparison to the 8.0 mm diameter shaft as they are close to the same size (the "Taimen" gun uses only metric shafts and they have to be matched to the gun which is specifically manufactured to use one size).
So one way of checking that an inner barrel vacuum was present after cocking the "Taimen" gun would be the shaft not falling out of the muzzle, provided that the entire projectile weight was under the maximum that could be supported by the vacuum (there may also be some help with friction from the muzzle rubber seal acting on the shaft). An advantage, although probably small, is the shaft does not have to jerk free of the piston when it leaves the gun as there is no "conic friction" holding it in as is the case with conventional tapered spear tails. According to the forums the spear tip could catch on something when grubbing amongst weeds on the bottom and then the shaft can be pulled forwards in the gun, but it would be a simple matter to push it back in, provided that it was not pulled clear of the muzzle seal (which is fixed in place) and did not admit any water into the gun when doing so.
Possibly standard pneumatic spearguns converted to pneumo-vacuum operation could dispense with their tapered spear tails (using cylindrical tails instead) and just rely on this vacuum effect to hold the spear in the gun. The mass of a Cyrano 110 cm threaded end spear of 7 mm diameter is 330 g. with another 35 g. for the tip, which gives a combined mass of 365 g. and as a mass of 398 g. can be supported (see above), so there is a small margin, provided that the vacuum in the inner barrel is good.
