I have several synthetic stocked weapons, not just my muzzleloader. I fill the butt stock with the expanding foam insulation. Just spray it in, do not fill it full cause this stuff expands like crazy. Helps deaden that hollow sound. I have not tested it, and hope not to test it, but I am thinking if the gun got dropped overboard even if it did not float completely at least the butt end might stick up enough to find it in dark water. $.02