Here is a great product that works great on the cap end of a sidelock/percussion cap rifle: http://octobercountry.com/products3.php?productid=539 It uses a special nipple with an "O" ring that seals to outside moisture when you slide the brass cover over it. I have used them on my T/C renegade and never had a hangfire/missfire in the rain. It serves as a safety too!

As stated, I don't think you will have a problem with the 209 inline. I hunted this September in the rain for 4 days and my Omega with nothing but a"muzzle mitt" over the barrel and itwent off with no problem.

However, also remember to never bring the gun in a warm cabin at night, that can cause condensation inside the barrel. Leave it in the truck or tent.