Hi Vincent,
In certain pumps, the petrol pistol barrel will just slip in without any problems. In your case, once the tip of the barrel rest on the openning of the mouth of the tank, hold the pistol firmly with the tip pressed on the mouth of the tank, twist the pistol slowly and gently to the left or right. It will eventully slip in and you can start unloading the fluids inside.
Funnily enough, once filled, no twisting action needed to remove the tip from the mouth.