A quick web search yielded these answers:
Frequently Asked Questions (Cartridgecollectors.org)
Is old ammunition safe to shoot?
We cannot answer that, even if we see the ammunition. While it may be a desirable collector item, it may be useless for the shooter. Primers can lose (or increase) their sensitivity, propellant charges can deteriorate and fail to burn or burn improperly. Cartridge cases may become brittle and split or rupture when fired. Bullets may become stuck in cases from reaction between different types of metals, or they may change physical dimensions from oxidation or chemical reactions. Age, storage conditions, temperature extremes, and original manufacturing practices may vary greatly and two similar cartridges stored under different conditions may or may not be equally suitable for use. Old ammunition should be saved only as collector items, not for actual use. It is better to spend a few dollars on new ammunition than to risk a misfire or worse which might damage a valuable gun (or the shooter) by using old ammunition of questionable condition.
Google Answers Disposing of old shot gun shells
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Most law enforcement agencies have a policy in place that provides for
the collection and destruction of dangerous materials such as
ammunition, biohazards, chemicals, etc. Call your local law
enforcement agency and explain that you have some old DAMAGED AND
POSSIBLY UNSTABLE ammunition that you would like to turn in for
disposal. They will, most likely, accept it and may even come out and
collect it directly from your home.
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Well, yes, but your research still didn’t answer the question: “Is (was) this 155 mm bullet (er, cannonshell) “safe to shoot?”
After all, one would not to squander unnecessarily the rare opportunity to shoot this down at the ole skeet range, would you?