 Tumbling occurred when the shell failed to take the grooves inside the gun barrel or when the spin wore off in flight.The caliber of the 20-pounder Parrott was the same as that of a smoothbore M1841 6-pounder field gun, which fired a round shot with a diameter of 3.67 inches.Its rifled projectile was heavier than the 6-pounder's shot because it was elongated.