A 75mm Howitzer shell. The casing has mostly corroded away, but you can make out these letters: EXP. 9-25-98.
Weight: 18143 grams
These large, heavy shells can be found in one of the Special Encounters. On rare occasions, you might find one elsewhere in your travels. Considered unusable as live ammunition, they might have some other use. See the quest to Howitzer Shells (Quest).
3 can be found in the Retrieve the Gun Runner's package quest, or 5 in the rare bunker special encounter.