I have a t/c thunderhawk in 54 cal. and do cover the barrel with a small zip lock plastic sandwhich bag and as far as the breech area the thunder hawk breechplug is not removeable and I do apply knight grease to the tread of the nipple and use the no#11 RWS 1075 pluc caps,these cap fit so tight over the nipple ,no water or moisture enter at all.from the breech area.Ialso have to say the only mis-fire I ever had was when I went to musket caps,musket caps are hotter but they don't fit as tight over the nipple area letting moisture into and threw the nipple.
I have to use the edge of my knife at the end of the hunt to remove the nipple otherwise that RWS caps will not come off.I have been in rain, cold sleet, snow with this thunderhawk and he gun has never ever lwt me down.
I also have now the CO2 discharger now I pick up a while back that is made by t/c (no longer made) to discharge the load without firing the gun ,man it works great,make sure the barrel is always pointed in a safe direction when using the dis-charger