For rifles that get used a lot, toughness is critical. In the modern gun world, that means that the first choice in stocks tends to be synthetics. While they are far tougher than traditional wood stocks, synthetics are not completely indestructible. They can be cracked and even broken. The most common failure point in a synthetic rifle stock tends to be a crack across the thinnest portion, directly between the receiver and the butt stock. This handgrip area, called the "wrist," can be broken in a number of ways. Fortunately, repairing it is a simple procedure.
Unload and clear your weapon. Double-check the chamber both physically and visually, to ensure that the weapon is clear.
Place the weapon in a padded-jaw vice and tighten the vice around the stock of the weapon, just in front of the trigger group assembly. If you do not have a gunsmith's padded jaw vice, a normal vice can be used, as long as the jaws are padded to prevent damage to the stock.
Use a small piece of 40-grit sandpaper to sand the rough spots of the inside of the damaged areas of the crack. While the inner portions of the stock crack do not have to be perfectly smooth, they do need to be cleaned up and roughened slightly to ensure the repair will stay in place. Clean the crack afterward with a clean, damp cloth.
Use a small piece of fibreglass gauze and soak it thoroughly in liquid fibreglass resin filler, like the type often used in automotive body repair work. Roll the wet fibreglass gauze tightly.
Use the head of a flathead screwdriver to shove the rolled gauze tightly into the crack on the rifle stock wrist. The rolled gauze needs to be fitted as tight as possible into the bottom of the crack, to ensure that the crack is repaired permanently.
Fill any additional space remaining in the crack of the stock using more of the liquid fibreglass resin filler, until it bulges slightly over the level of the surrounding stock.
Allow the liquid fibreglass resin filler to dry. The duration of time needed to dry completely will vary depending on the particular brand used. Typically, in order to allow the resin to dry completely at the bottom of the crack, it is a good idea to allow the repaired stock to dry for at least 24 to 48 hours longer than the time recommendations listed for the product.
Sand the bulge of the dry fibreglass resin filler with a 40-grit sandpaper until it is level with the material of the original surrounding rifle stock.
Paint the entire rifle stock, including the repaired spot, with a simple primer spray paint or the commercial stock finish of your choice.
Because fibreglass resin filler dries hard, it is easy to do lasting damage to your rifle stock. Use caution. Use appropriate eye and respiratory protection when working with fibreglass products. Firearms can be inherently dangerous. Please seek competent professional instruction prior to handling any firearm.