AGM

[Andrew McHarg]

The Oxford English Definitions suggest they are similar words:

Shall:

Expressing a strong assertion or intention

Expressing an instruction, command, or obligation

Must:

Be obliged to; should (expressing necessity)

Expressing insistence

I agree that "shall" suggests the possibility that what is intended may not happen. But so - too - can "must".
Certainly the word "shall" carries the same direct and uncompromising tone as "must".