An interesting question, Double D. Why not simply cast it along the lines of the pressure curve? Rodman did and his guns achieved notable success. From Dahlgren's first patent on 6 August 1861, No. 32,983, we learn that he was very aware of pressure curve theory.
"A cast-iron gun cast substantially according to the rule ... whereby the quantities of metal disposed in the different parts of the gun are proportionate, or nearly so, to the relative degrees of strain exerted by the force of the exploded charge at those parts respectively."
This explains the "soda bottle shape" of his guns, which is a bit different than the Rodman curvilinear shape. Dahlgren's design digressed from pure theory to have a cylinder for the purpose of aiming over a regular surface at point blank distances should the sights be disabled.
"Rather than striving for flawless iron when molded into a cannon, he strove to shape the raw gun block castings to maximize the homogeneity and quality of iron thoughout. A weapon machined from superior metal could contain nothing but superior metal. He observed in his second patent, No. 32,984 of 6 August 1861."
"To produce cast iron of the greatest strength, it is necessary that the molten metal, after being poured into the mold, should cool with evenness and regularity throughout the entire length of the gun, to effect which it is indispensable that the diameters of the casting should vary as little as possible, and that the external surface should be uniform and free from prominences of any kind, or at least from such as have material volume; if not, the mass cools and solidifies first where it's diameters are least, and afterward, where they are greatest, and the parts which cool last shrink from the parts which cool first, thereby straining the metal and diminishing the strength."
"Rough Dahlgren gun block castings provided the most excess metal where the finished diameters were to be smallest." “A post-War letter states that IX-Inch shell-guns “have always been cast solid and bored out.” It is a tribute to Dahlgren’s monumental design and gun foundry achievement that there is no mention of IX-Inch shell guns in the critical Senate regarding ordnance failures.”
So, there you have it, the machining was done to lighten the piece for easier handling, but only enough to comply with pressure curve lines. The extra cast iron was provided in the gun block to obtain the most homogeneous and superior metal that was possible. All information paraphrased and quoted in this post came from the excellent book titled, The Big Guns by Olmstead Stark and Tucker.
Tracy and Mike
