Hi,
Joe, thank you for that bit of knowledge about proofing. I learn something new every time you offer up those tidbits.
Rich, I am a little confused. Are you thinking of having the barrel stocked as a British gun with a Spanish barrel or are you thinking it was originally installed on a British-made gun? If the former, then it is more likely it would have a hook breech if stocked by a first quality British maker during the mid-18th century. Of course I am sure one could find plenty of exceptions. Alternatively, if you think it came from a British gun originally, what gives you that idea? It may never have been used on anything other than a nice Spanish gun, many of which had standard breech plugs.
With respect to Jim's question about length, size, and aging, I dug into my copies of Spanish Guns and Pistols by Neal and Espingarda Perfeyta. Neal states that Spanish barrels were usually about 42 inches early in the 18th century but many were made shorter during the latter part of the century. However, in EP the authors use an example of making a barrel about 1 meter in length to demonstrate portions and proper weight distribution. Although, given their descriptions of barrel making, the norm was clearly for making longer barrels but shorter barrels must have been made as well. The gunsmith manual was written during the last decade of the 17th century but published in 1718. Also both books note that first-quality Spanish and Portugese barrels have gold lined touch holes after about 1700.
dave