Stone cannonball?

[Ed]

You might get a vessel of the appropriate size, pour in enough water to cover the object (before you put it in). See what the level is by making a mark on the side.

Insert your ball and see how much water is displaced. Calculate the volume of the displaced water.

Like this ... diameter of vessel is 6 inches.

When you immerse your ball the level goes up 2 inches.

Ergo the increase in volume is expressed by π r² h
Pi = 3.14
r=6/2=3
rsqr=9
H=2

Soooooo the volume of your ball is 3.14 x 9 x 2 = 56.52 cubic inches

Now weigh the sucker. Lets say it is 4 lbs

The density is 4/56.52 = 0.07 lbs/cubic inch.

Now hunt around for a density table .... here is one

Seems this particular ball is magnesium.

I have just provided you with hours of fun!!!