How things change, or maybe they have not!

[popgun pete]

My browser for some reason flipped me right back to page 27 and I then read this thread:
https://forums.deeperblue.com/threads/new-dry-pneumatic.32885/
The issues are familiar, although the posters may have moved on to other things as I cannot recall their posts here in recent years.

 

[Pinniped72]

It makes you wonder were all the posters are? Hopefully safe and well and still diving

 

[popgun pete]

Did you notice that in post #12 "Iyadiver" mentions a pneumatic gun with a 23 mm diameter piston that operates at 25 bar! From what I can tell, after reading it right through a couple of times, is that the experimental speargun is cocked by pushing the piston back with water using a hydraulic pump fitted at the muzzle. I assume that the hydraulic pump is detached once the piston is locked by the trigger mechanism and then all the water is tipped out of the inner barrel. A spear with an "O" ring seal at the muzzle is then inserted into the cocked gun, the seal being required to keep water out as otherwise the big piston would have to push out a large column of water with the shot. I think running the spear down into an already cocked gun would be potentially dangerous, so the order of loading could do with some revision, but it is an interesting concept. The same principle is used in hydropump cocked, hydropneumatic spearguns, but there the travelling piston is in a sense once removed as it is not being pushed backwards inside an inner barrel, but forwards while running on the outside of the inner barrel in the form of an annular piston. An interesting comparison all these years later is the "Dreamair" which also uses a big travelling piston and relatively low air pressure to drive a spear from the gun, but via a system of cables and winding drums that operate like a very high speed winching system.