Your sketch of the battery seems accurate. Information on the guns is that early models of the light 12 pound gun, commonly referred to as the Napoleon, cast by the federal goverment had handles. The last of which(with handles)was delivered in December 1861. However, no federal purchases of any Napoleons were cast at the Tredagar foundry in Richmond, Virginia. This foundry did cast bronze Napoleons for the Confederate goverment but not until 1862. Tredagar did cast some 12 pound bronze field guns in 1861, of the model 1841 type. This model had handles with rectanglar attachment pads Some of these were for various states, Mississippi included. Most likely, the guns you seek info about were from this batch of 12 pound bronze field guns cast by Tredagar in 1861. They are described as "heavy" since they weigh just over 1,700 pounds. Guns of this type were not intended for field use but for garrison, seige or non mobile use. The material used is bronze which is an alloy of nine parts brass and one part of tin. Brass was a basic metal used to make the alloy.