using stirrup pump

[angrybear]

It's really simple, if the gun has 100bar in it then to fill it further you need over 100bar in the hose to push open the valve, so first you have to pump the hose full to 100 bar.

Once the hose pressure is equal, the next few pumps will push the hose pressure higher than the gun pressure & that extra will push the gun valve open,
as soon as the pressure in the hose & the gun are equal a tiny little spring inside the valve will close it,
the next pump stroke again makes the hose pressure higher and it all happens again, & so on with every stroke of pump, until both the hose & the rifle contain the max pressure.

You then vent at the pump so the pressure in the hose drops & the spring & internal pressure in the gun holds the valve closed to stop the air inside the gun escaping.

If you have a scuba cyl to fill the gun, the higher pressure in the cyl will push the gun valve open and will fill the gun in one push,
but as the cyl empties there will come a point where the cyl contains less pressure than the rifle max at which point the two pressures will equalise and that's it until the cyl gets refilled.