I have a theory. If ammunition cartridges are used in casting,
especially center-fire cartridges, one benefits from the fact
that the primer is made of springy (read phosphor) bronze.
Phosphor is sold as a degasser, if I remember correctly. Anyone?
This only applies to brass cartridges. There are aluminum rounds
out there now, and most shotgun shells have gone to a plastic
body, good for an icky mess in the crucible.