Welcome aboard, Patrick. Do you plan on making an exact (scale) reproduction of the proper carriage for an 8-inch siege howitzer, and are you getting the Model 1841, or the Model 1861 siege howitzer (both models were used during the Civil War)? Both models had the same diameter trunnions, and distance between the rimbases, so both were mounted on the U.S. 24-PDR Siege Carriage.
SIEGE CARRIAGESWhite oak was the wood used to make the real period siege carriages.
SIEGE CARRIAGE DRAWINGS No. 80.
Model 1841 8-inch Siege Howitzer mounted on a Siege Carriage.

