John,
 
We have had several discussions on the subject of cleaning shells since last
fall. You may want to look them up in the CONCH-L archives.
 
The traditional method of killing land snails, especially if you need the
soft parts for dissection, is to drown them in water. You put the snail in a
jar filled with water to the brim, then you place the lid on without leaving
any air space. The snail dies extended. I have never done this but I read
about it all the time. Somebody correct me if I am wrong.
 
A.