Most commercially available shotgun shells come in boxes of 25.
Let's hope you don't have shotgun shells filled with nitroglycerine! Shotgun shells are limited in power because shotguns are not cannons, They are very light weight weapons. Gunpowder (any of several types) lasts virtually forever if kept dry.
Air soft shotgun shells are the size of 12 gauge shotgun shells.
Find someone who reloads shotgun shells. They will relieve you of your inventory.
Yes, there are as well as people who collect other types of cartridges. Both the loaded type of shotgun ammunition and the empty 'hull' of a shotgun shell. If it exists, there is probably someone who collects it.
No
You can use nothing but 410 shells in a 410 shotgun.
NO
Shells
No.
Yes, you can.
Generally "Pellets" or "Shot" contained in "Shells". There are also solid shot "Slugs" similarly contained in "Shells". I'd use Shells ie "8 shotgun shells", unless you are specifying exactly the type.