If your buffer tube has become loose, it is very easy to tighten it back down. Follow the steps below to correctly tighten your buffer tube.
You will need a long Phillips #2 or #3 screwdriver. Remove the battery from the stock if it is currently installed.
First, using the the screwdriver, loosen, but do not completely remove, the screw inside the base of the buffer tube. Then move the buffer tube and stock a little to the rear so you have some room to work with.
Next, move the castle nut towards the sling plate and receiver by turning the castle nut by hand. No special tools are required.
The castle nut should be adjusted to the point that it goes a little beyond the end of the threading on the buffer tube. The idea is that the castle nut makes contact against the receiver and sling plate before the buffer tube itself does. The following photo shows an example of the castle nut going past the end of the buffer tube:
Finally, tighten the screw inside the buffer tube with your screwdriver. Tighten the screw until it is snug ("hand tight"), then an additional 1/4 turn. Your buffer tube is now correctly tightened.