Kayuk module

Check out this module being produced for the "Kayuk" speargun and other brands of pneumatic spearguns. You cut off a length of your barrel tube (tank) and then insert the module to replace it. This then provides a pressure gauge on your gun and also allows accessories to be fitted in the other three slots on the module. Pressure gauges on guns are not new, the Technisub "Jaguar" had one located in its rear tail cone. The Russian "Neptune" has a pressure gauge in the mid-section where the handgrip is mounted Hopefully the pressure gauge does not leak and depressurize your gun, but at least you could see it happening.

Various tank diameters and inner barrel tube OD's are catered for with three versions of the module..

https://kayuk.by/shop/module

Here is the module being used at various positions on a gun; the tank needs to be cleanly cut at right angles to the tank tube axis say by using a parting tool in a lathe.