The dimensions of the Inalex "Alpha C1" gun are as follows:
Inner Barrel ID: 14 mm; hence so is the piston which is tail-less and is held by the vacuum behind it situated between its rear end and the front face of the releasing valve when the gun is cocked to shoot (that is how all the releasing valve guns work, even the hydropneumatic versions).
The "Alpha C1" models available are, in terms of gun overall length (barrel length): 77 cm (60 cm), 97 cm (80 cm), 117 cm (100 cm) and 137 cm (120 cm). I assume the "barrel length" is the working course of the piston, so to obtain the working course of the piston you subtract 17 from the overall length of the gun in centimetres.
The grip handle can be placed at whatever position you want on the gun when you order it. The main change is the length of the pull rod connecting the remote trigger located in the grip handle to the rear releasing valve mechanism situated in the butt end of the gun.
Maximum Operating Pressure: 30 Bar
Usual Operating Pressure: 18 - 20 Bar
Loading Effort: 22 - 25 kgf for the above air pressures
The "Alpha C1" gun uses the inner barrel as the hand pump with the muzzle set to "air pump" mode. The compression ratio of the hand pump is deliberately limited by the pump stroke to produce only 30 Bar so users cannot go beyond that pressure regardless of how many times they pump the gun as a safety measure. The gun's shooting safety is activated by screwing the rear butt power controller right up to imprison the releasing valve, so there is no chance of a shot in this situation, again an emphasis on safety which Alex says was an important factor in his design. Hence no safety lever to flip back and forth and forget just where it is at as on so many other spearguns, but the gun can be rendered safe while you conduct your shooting line wraps, which is what you want a speargun safety for. Then you set your desired power level and commence the hunt.
The "Alpha C1" gun can be muzzle loaded with repeated spear insertion strokes until you are able to insert the spear all the way into the gun.
Once you have pushed all the compressed air behind the piston through the releasing valve's front end valve the piston stays back except for a very short rebound action which occurs on all releasing valve guns (I know this from using the GSD trigger valve-operated guns, it is just residual air expanding from a tiny, highly pressurized volume back to ambient pressure).
Note that with a 14 mm inner barrel you gain more propulsive force for any given chamber pressure, which of course you have to load against unless you opt for a lower pressure to compensate. A similar releasing valve gun was the GSD "Katiuscia", it also had a 14 mm ID inner barrel and was a floater, unlike its "Dynamic" predecessor. As far as I know the GSD pneumatic gun was the only releasing valve, rear handle gun available in the West and with its departure there was nothing to replace it. The next GSD gun produced was the "Punto", but it was a mechanical catch, ball sear gun despite its strong family resemblance to its GSD predecessors.
From what I have seen of it the "Alpha C1" looks to be well designed and finished with very good attention to detail, just check out this latest image of the muzzle. The white grip gun handle photos are ones I saved earlier when the gun first appeared in print.
An action shot: