what Hank says is correct and one must be very careful,NOT to "crush" or mark the top edges on BOTH sides, as well as keeping it "uniform", you can deform it or make it out of square, quite easily....'if you do NOT want to attempt to "adjust( refit) the grip frame, you can put a piece of electricians tape or a match book cover or some such, thin piece of cardboard, and place it in the notch ,up front that hooks into the under side of the receiver,,, then tap the upper assembly in place,,,this works and will keep it tight until the next time you remove the upper receiver,then do it again......it works.