(In the meantime keep it submerged in freshwater)
Add 3 or 4 tablespoons of lye, like Red Devil Drain Cleaner and mix it in water. Add a piece of scrap metal, not touching the projectile.
Get a manual battery charger, attach the + to scrap, - to object; it might be easiest to drill it and put in a screw for good contact. It needs to cook for 3, 4 months.
The salt in the object will form crystals when it dries, and crack it to pieces-so get as much out as possible. A black sludge will form on the scrap too,
that is carbon coming out along with the salt. Rust, dirt will form on water-it gets very dirty, change to new solution. As time goes along the encrustation
& mineral will mostly fall off; you can help it along with a hammer/wood mallet. [ the more time in a slow soak electrolysis-the more chance it stabilizes.]
But go easy, it will be soft from the carbon loss. (Once it is rid of the encrustation check it for a plug/fuse-if one is present, its a shell and needs to be deactivated.
The black powder when dry will go off. If you contact Police/Fire-they will seize it!-so try Civil War Forum, or websites for help)
Once clean, soak it in fresh water to help remove the lye. If you now know it is safe [deactivated/or a bolt (solid)] dry in oven, & once cooled it must be coated
with several coats of polyurethane or tung oil-I prefer the later. (some people use wax, but it is tacky/sticky)
That's about as much as you can do-but some, still crack up into pieces.
So do you think still have black power in it????? it can explode???