The only way it could work, would be using the ice as a fuse, rather than filling it into the mortar. Something like a device that keeps the shell in the end of the tube and drops it when the ice melts. The most primitive way would be a nylon string attached to the shell with the other end of the string frozen in an ice cube, with a piece of metal or wood with a hole smaller than the cube which keeps the string strained.
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