Its shape is "somewhat" similar to some pre-civil-war era cannon "Proofing" projectiles... which were made ONLY for testing a newly-manufactured cannonbarrel's strength, by firing a heavy solid-bodied projectile. In my 40+ years as a civil war artillery projectiles digger, dealer, and "scholar" I've never seen a "Proof" round with that particular shape. However, due to the similarity, I suggest measuring its diameter as precisely as possible, with a Digital Caliper, in hunderedths-of-an-inch... then see if the measurement matches up with any of the known diameters of cylindrical civil war artillery projectiles. (Such as, 2.92" diameter, made for use in a 3.0"-caliber cannon.)
