One important measurement is the nominal length, which was used as the technical length of the gun and is the length given in most old documents and modern texts written by cannon-knowledgeable authors. Nominal length is measured from the muzzle face to the rear of the base ring. The cascabel is excluded. I'm guessing the nominal length of that gun will be either 5 1/2 or 6 feet since I know nothing lengths of 4-pounder were in use in England during the 18th C. For iron land-service guns. I haven't looked up sea-service 4-pounders yet.
Would also like bore diameter. Please check trunnions ends for marks too. I'm sure you know the three sets of numbers on top of the breech give the gun's weight in hundredweights.
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