Have you tried pulling the lamp back a bit and see if that helps, might be your lamp is reflecting on your barrel being as its the stainless. Worth a try.
Pump your gun upto 100, then take it out and fire it until it's flat, and then some anorak will tell you how many shots your gun will fire on one fill. If it's fires the required amount of shots for a full bottle, then it's your pump gauge.