I found an old tin of caps a while back in a possible bag that had to be pushing 10 years or more. They shot off fine. The way you're storing them, there should be no problem. I normally by my caps, 20 tins at a time so I have them all year.

That is a beautiful rifle you have there. Like Sabotloader, they are my favorite rifle. I guess that is why I own so many of them. I still want to convert one of mine to a flintlock and then get an big bore barrel for it... They are a great rifle. You did a wonderful job on your stock...