When you take these 520s apart, reassembly can be a tedious task.
As an example, the leaf spring under the hammer spring has two legs and they tend to slide under the trigger and under the slide lock. One leg should fit atop a shelf behind the slide lock and the right leg fits atop the back of the trigger. When you reassemble the leaf spring, the leaf spring's legs tend to slide under the trigger and slide lock. They should be lifted up as you slide the spring forward, or muscle the legs up after the spring is in position. This usually causes the action to NOT lock up, or freeze the action in the closed position.
Another oversight is, depending on which model of trigger group you have in your gun, (there were several variations), your slide lock release spring may be installed incorrectly. The long end of that spring goes on the outside of the left pin under the slide latch looking at the left side of the receiver. Not between the two pins.
And yes, the shell lifter will have tension on it after you pull the trigger and loose during lock up. So, I don't think you have an issue with the gun.
