First off, I am presuming you mean a Remington 870 not 780.
What you are referring to is called "Cruiser Ready". You want a loaded magazine tube, an unloaded chamber and the ability to quickly and easily "rack" the action to fire.
First understand that you must have the action completely closed to load the magazine. If the hammer is cocked, the bolt will be locked forward and the slide release lever must be used to rack the action. The hammer must be in the fired (uncocked) position to rack the action w/o using the slide release. To get into the proper condition follow these easy steps.
Ensure the gun is completely UNLOADED. Close the action, put the safety on FIRE, point the muzzle in a safe direction and pull the trigger. DO NOT rack the action. Leave the safety in the FIRE position.
Hold the slide forward (in the closed position) and load the desired number of shells in the magazine tube.
When you need to shoot, simply rack the action back and forth to load and fire.