The term seems to be common usage on the milsurp forums, correct or not, for the sealed cans milsurp ammo often is packaged in. A very few of the ammo cans do have an opening strip, much like actual Spam cans. One ammo dealer also refers to these ammo tins as "sardine cans".
I did run across a how-to on opening these cans:
http://www.russian-mosin-nagant.com/ammo.htm