The guys took care of you as far as the answer to your question goes. Here's just another point. I used to have few shells (about 10 rounds, mixed with flares) that rode with me on kayaking trips. Sometimes I would take my 870 and sometimes it would stay home but shells were in one of my vest pockets and they were exposed to rain, dirt and anything possible. SOme of them were in my vest for 5-6 years. When I got them out on the range one day they were all fine except one which didn't fire. The one which didn't fire was a turkey shot with brass part that is usally shorter than regular buck or slug shot. I didn't have any extra shells to test if this was due to time it spent in harsh conditions. I know from my dad when shotgun shells were made out of paper cardboard tubes that prolonged moisture was a problem but not on plastic ones. Some guy at the range also told me that good way to test your ammo is to grab the butt and top, give it a little brnd and see if it's loose. Don't know what it means thou. Also in a kayaking magazine I saw advise on using wax to seal top of the shell for extended water trips.

I still have rounds I bought 10 years ago that fire fine. SOmetimes I save better ammo when going "plinking" and than I don't get a chance to use it. I think the life depends on the proper storage.