I have taken the luger apart, but now that I put it together the upper piece (the receiver) seems to not be locked correctly? It is loose? How can I fix it?

It might be that the hook that connects to the mainspring is not staying in place when you re assemble. The T hook must drop into the well and be in position to grasp the top hook on the mainspring and stay there.

This is accomplished by putting your right hand thumb in the trigger guard and grasping the pistol by wrapping your fingers over the top of the barrel. This will enable you to hold the top of the pistol to the rear while holding the pistol rear down towards the floor.

The T hook that attaches to the toggle will then fall into the correct position to grasp the mainspring hook without falling forward. While holding the top to the rear you will feel the tension of the mainspring. Using your left hand pick up the sideplate and slip it into place. Then swivel up the sideplate locking piece and its done! The pistol should be held in place and cycle correctly.

Jerry Burney