The cast iron shell, similar in shape to a Minié ball, had a thin lead coating which made it fractionally larger than the gun's bore and which engaged with the gun's rifling grooves to impart spin to the shell. This system had just been developed by Martin von Wahrendorff and Giovanni Cavalli in Sweden. This spin, together with the elimination of windage as a result of the tight fit, enabled the gun to achieve greater range and accuracy than existing smoothbore muzzle-loaders with a smaller powder charge.