Ya what Kelly said.. they are screw in wgts that come in several wgts. you can stack them. They also sell a long allen wrench so that you can add / remove from the back. If you use pin bushings, (recommended, at least by me if you target or 3d shoot), you do not glue them in. GT says to use plastic bag or teflon tape for a frictional fit. Once set you may want to consider using light duty loctite on the wgt's and a light glue for the pin bushings. you can also add wgt to the rear to keep your correct.
Of course you may get what you want by just adding point wgt?