Although GunSkins is designed to stretch, there is such a thing as taking it too far. Over-stretching causes the color of the print to turn white as it thins out. The adhesive will likewise lose strength because it's covering a larger surface area than intended. If this happens, peel the vinyl and let it hang. Use a swaying motion with a heat gun on the lowest setting and watch as it pulls back in and reverts to its previous shape, ready for another go.
It has an internal memory and the heat will allow it to reset to its initial shape. When left to cool, the vinyl will harden and be ready to lay down properly. Be careful to avoid stretching too far around pistol grips, overlapping on the buttstock, or around the bell-shaped lens of your scope. It's better to use a large piece of vinyl and trim away the excess than to stretch a small piece to cover a larger area.