I personally wouldn't put a silicagel sachet anywhere near one of my rifles. Used over a long period of time, Silicagel can cause woodwork to split due to drying it out too much. Those Napier versions are supposed to be better...
Neither would I put anything over the muzzle, it's better to let air circulate through the barrel ( preventing any moisture build up ), leaving the bolt open but not cocked on PCP's or break actions in an open state is the best thing to do.
Craig
Put on heading 270, assume attack formation