I'm guessing the barrel is getting hot. When you fire a gun, a very hot explosion goes off and sends a column of blazing gasses through the barrel. The bullet also rubs against the rifling, which is what gives it spin and therefore accuracy. There's a whole lot of friction and heat being generated in there. And when steel heats up, it gets flexible and expands a little bit, loosening everything up and whipping around. You also might be taking followup shots too quickly. Try again with iron sites and see if you have the same issue with your first shot being perfectly accurate but with the subsequent shots growing increasingly inaccurate. If the issue persists without the scope, then it's either your technique, or the gun getting hot. If not, then it means that the scope is losing zero. You said that you're using a light version of the gun, so it is quite possible that the barrel is heating up and losing accuracy. Try again with a heavier-barreled gun. If the problem persists, then it's your technique that is at fault. If the problem goes away, then the issue lies with your gun's barrel heating up. A bad workman blames his tools, but a good workman knows what he needs to get the job done properly.