Hi SGE
Neat find. Without seeing a picture (and I am not an artillery expert), I would agree with your gut that given its size and depth of location. it is older than Frement. However, if it has a place for a fuse, I do not think it is less likely to be 300+ years old.
Wish I could offer more. I bet there are many artillery projectile pages on the web to help ID it.
cheers
tps