The rear end of the action spring is only kept from flying free by that shoulder on the receiver, at the rear of the magazine tube. You need to compress the action spring until it's being held by your hand instead of that shoulder.
Next, the slide assembly must be moved rearward slightly (1/4" or so), and lifted free of the bolt and receiver. When you do this, the operating handle (bolt handle) should fall free of its notch in the slide. They don't always do this, but they're supposed to. So perform this operation over a table, to catch it when it falls out.
It's tricky to compress the spring by pulling it towards the front of the gun, and then move the slide to the rear to remove it. But it can be done, I assure you.
Now that this is done, there's nothing to keep the gas piston from falling out of its cylinder, so be careful not to lose it!
