There were NO “shells” used on NJ during ANY part of the 1776 war, though much fighting through the state.
Very little shrapnel (powder loaded shells) anywhere in the 1812 (it was just invented in 1806 in the Spanish - Napoleonic wars) war - but, little fighting in NJ though.
Perhaps loading practice in NJ in the Civil War, but no fighting there.
Now, about this rusted brass .... 8<)
The pix posted is NOT the subject shell, but the earlier find. No pix of the subject shell.
Even a Camp Vrednenburgh existed for the Civil War on or near the grounds encompassed by Monmouth so undoubtedly that could be a possibility. But I cannot believe they would use metric. Hence I think should be even later. Posiibly some of the last.
Unless they titally misnomered it and it is nit a “ball” at all.
Can an iron cannon ball be x-rayed? I have one in the basement. I have never dropped it. How would I know if there is a charge in it, or that it is just weight?